Fence Post Replacement Recipe
Tools:


Safety equipment


Eye protection (such as wrap-around sunglasses)
Hearing protection (such as ear plugs or ear muffs)
Dust mask

Facilities


Dry, well-ventilated work area (for working with wood preservative and/or caulk)
Sink (for cleanup)

Measuring tools


Level (such as a meter-long spirit level)
Tape measure or carpenter's rule
Long string or twine (a couple feet longer than needed to stretch between the nearest good fence posts)
Pencil

Containers


3 five-gallon buckets, with lids (for holding crushed rock and waste concrete)
Tarp(s)
Caulk gun (The Spruce explains two types of caulk guns.)
Reusable cap for caulk tube

Digging tools


Narrow tamper / digging bar (such as a 16-pound Collins Axe tamper / post hole digger)
Shovel
Trowel (such as a bulb planter)

Fastener adders and removers


Claw hammer (such as a 16-ounce claw hammer)
Miniature crow bar (such as a 14-inch Estwing nail puller)
Phillips-head screw driver (if any Phillips-head screws need to be removed from the fence)
Flat-head screw driver (if any flat-head screws need to be removed from the fence)

Cutting tools


Scissors
Knife (such as a carpenter's knife, snap-blade knife, or Exacto knife)
Saw (such as a hacksaw)



Disposable tools


2 Disposable work surfaces (such as corrugated cardboard boxes)
Disposable paint brush (if your fence post was cut after being pressure treated)
Disposable finger bowl (such as a cut-down yogurt cup, for handling the edible mineral oil)
2 Disposable pairs of gloves


Materials that do not become part of the fence


Lava soap
Dish soap
Edible mineral oil
Paper towels
Small plastic bag (for disposing of trash)
Painter's tape (such as 1 7/8" painter's tape)
Refreshments (such as water and/or Gatorade)


Materials


4" x 4" (nominal = 3½" x 3½" actual) pressure-treated fence post rated for ground contact, approximately 40% longer than the above-ground height of the fence post.  (Many lumber yards carry fence posts in 2-foot increments, such as 6', 8', 10', and 12' long, and will shorten the posts if requested by the customer.)  For example:  


above     total       hole
ground    length      depth
8'6"   -&gt; 12' long    3'10"
7'     -&gt; 10' long    3'4"
6'     -&gt;  8'8" long  3' (8'8" is often cut down from 10' long)
5'     -&gt;  7' long    2'4"
4'     -&gt;  6' long    2'4"

Liquid wood preservative (if your fence post was cut after being pressure treated, such as Copper Green Brown)
1 pair of Fence Fins
30" Small diameter rubber tubing (such as 5/32" ID windshield wiper vacuum hose)
Adhesive for bonding rubber to metal (such as butyl rubber caulk)
36 1½"-long galvanized nails (such as heavily galvanized #8 nails)
8 small metal angle brackets (such as Simpson Strong Tie part A21.  Use another 8 if you need to apply angle brackets to the other ends of the fence panels.)
5 - 15 gallons of 5/8" minus crushed rock (exact amount depends on how much concrete is removed from the fence post hole and discarded)
The dirt that you will remove from the fence post hole.
0 - 2 gallons of extra topsoil, if needed.  (For example, a Special Garden Mix.)


Steps


If required by state or local law, Call Before You Dig, at least two full business days ahead of time.  In many U.S. jurisdictions, this phone number is just 8-1-1:  as a free service, the local utility companies will graffiti your yard near where they think their underground utility lines are buried.  (You can request that they use flags instead of spray paint, but they will probably spray paint anyway.  Eventually the spray paint will wear off.)


(Steps 2-5 make sure that all surfaces of the lumber are preserved.)


Wear a disposable pair of gloves when handling the wood preservative.  The wood preservative is toxic, and hard to clean off.
If your lumber was cut after it was pressure-treated, apply wood preservative to the cut surfaces of the lumber.  Follow the word preservative manufacturer's instructions.  In particular, perform the work in a dry well-ventilated area, and use a temporary work surface to catch any drips.  Use the disposable paint brush.  Copper Green Brown is toxic, smells nasty, and creates stains.  The items treated with Copper Green Brown will smell nasty for weeks (but eventually the smell fades away), and the brown color is not necessarily permanent.
If you applied wood preservative, allow the preservative to soak in (per the manufacturer's instructions) before installation.  Copper Green Brown needs at least 48 hours to dry.
Paper towels, Lava Soap, edible mineral oil, and dish soap can help you clean up after working with the wood preservative.


(Steps 6-19 "glue" rubber strips onto the exposed top edges of the Fence Fins.  This reduces the chances of someone cutting themselves on the Fence Fins after the post is installed.)


Apply two short pieces of painter's tape to each Fence Fin.  Each piece should be on the part of the Fin that has the holes, along the top edge, touching the gentle bend that marks the edge of the part with the holes.  The purpose of the these pieces of tape is to avoid getting caulk on the part of the Fence Fin that will be nailed to the fence.
Use the scissors to cut the small-diameter rubber tubing into four pieces, each 7½" long.  (Each piece should be slightly shorter than the distance from a Fence Fin's sharp bend to the Fence Fin's gentle bend.)
Unroll the small-diameter rubber tubing, so that it is approximately straight.  If it has a residual curl, let the ends point upward.
Working on the other disposable work surface, slice through one side of the rubber tubing.  Make the cuts be straight lines, preferably on the shorter side of each tube's curl.
Wear the other disposable pair of rubber gloves while working with the butyl rubber caulk.  (Butyl rubber caulk is very hard to clean off of one's hands.)
Follow the butyl rubber caulk manufacturer's instructions to open the caulk.  (Typically, this requires using the knife to cut off the tip of the caulk tube, using the caulk gun's rotatable needle to prick the inside of the caulk tube, cleaning the needle with a paper towel, rotating the needle out of the way, and mounting the caulk in the caulk gun.)
Fill the inside of a piece of rubber tubing with the caulk.
Press the rubber tubing onto the top edge of the Fence Fin, between the sharp bend and the gentle bend.  The slit in the tubing should face the edge of the Fence Fin.
Apply a bead of caulk to each side of the tubing.  Use a gloved finger to "tool" (smear) the caulk along the line where the tubing meets the metal.
Use short strips of painter's tape to strap the tubing down to the Fence Fin.  The strips should go at curved parts of the top edge of the Fence Fin, a couple inches in from the ends of the tubing.
Repeat steps 12-15 to apply the other three pieces of tubing to the other three long top edges of the pair of Fence Fins.
When done, remove the caulk tube from the caulk gun, and apply a reusable caulk tube cap to the caulk tube.
Butyl rubber caulk will remain sticky for a long time.  Be careful when handling the Fence Fins.
Paper towels, Lava Soap, edible mineral oil, and dish soap can help you clean up after working with the butyl rubber caulk.


(Steps 20-23 record where the fence post was installed, so that you can put the new post in the same place.)


Tie the string to the tops of the nearest good fence posts.  Pull it tight, and make sure you know where the string is at the top of each post.  (This will help you put it in the same place later.)
Use the pencil to mark where the string crosses the fence post to be removed.
Use short pieces of the painter's tape to mark the edges of where the fence panels's slats and 2x4s touch the fence post.
Take down the string.


(Steps 24-29 dismantle the old fence post.)


Wear eye protection and hearing protection when bashing or pulling things with hammers and/or crow bars.
Use the claw hammer, miniature crow bar, and/or screwdriver(s) to remove the nails and/or screws that attach the fence panels to the fence post.
If you need to replace the fence post because it is rotten and/or leaning, the fence post may fall over once the panels are detached.  If not, try pulling out the fence post.
Use the trowel and/or shovel to peel the lawn back from the fence post, to reveal any concrete that the fence post was set in.
Use the tarp(s) to protect the lawn around the fence post.
If the fence post was set in concrete, repeatedly slam the sharp end of the digging bar into the post and concrete to break them up.  (This may make sparks.)  Alternate shattering the concrete with prying and digging out the concrete shards.  Put the buried fence post pieces in a pile on the tarp.  Put the concrete pieces in another pile on the tarp.


(Steps 30-43 install the new fence post, but do not attach it to the fence panels.)


Install the Fence Fins on the fence post.  


Place the Fence Fins so that their tops will be about 4 inches below ground.  
Before nailing, verify the position of the Fence Fins.  For example, an 8'8" fence post that will be 6' high should have the Fence Fins between 1'8" and 2'4" from the bottom end.
Before nailing, verify that both Fence Fins have the tubing on the top side.  
Follow the manufacturer's instructions in other respects.  
This will use 16 nails, because not all of the holes should be filled in.    

Dig out the hole, so that it is about three times the diameter of the fence post, and about 4" deeper than the bottom of the fence post.  (It needs to be wide enough that the dull end of the digging bar will fit between the fence post and the side of the hole.)
Extend the hole along the line of the fence so that the Fence Fins will fit.  These triangular extensions of the hole will need to be about 12" deep.
Fill the bottom 4" of the hole with the crushed rock.
Use the dull end of the digging bar to compact the crushed rock.  (Basically, beat on the rock until the rock stops settling.)
Reinstall the string, in the same place as before.
Place the fence post in the hole.
Adjust the fence post's height, so that its top is the desired height relative to the string.  (This might require adding crushed rock beneath the post, or removing crushed rock from beneath the post.)
Squeeze the fence post between the adjacent panels, to make sure that the post is in the right place horizontally.
Use the level to make sure that the fence is vertical.
Add another 4" of crushed rock at the bottom of the hole.  Compact the rock.  Repeat steps 36-38.
6" at a time, repeat steps 36-39 until you have added enough crushed rock to make up for the concrete you removed.
6" at a time, add the dirt you removed.  Try to put the dirt you got from the bottom near the bottom, and the dirt you got from the top near the top.  Repeat the compacting and measurements every 6".
If you need extra material, use the extra topsoil.


(Steps 44-49 attach the fence panels.)


Use the claw hammer, miniature crow bar, and/or screwdriver(s) to remove one fence panel slat from each panel.  Remove the slat that is next to the fence post.
For each fence rail, align two angle brackets on the fence post.  Align the angle brackets so that one is just below the top edge of the rail, and the other is just above the bottom edge of the rail.  Leave a gap between the brackets that is big enough that you can easily nail in the slat.  Put the long side of the angle bracket between the rail and the (missing) slat.  Put the short side of the angle bracket between the post and the end of the rail.
Remove the string and painter's tape that you used to help align the fence panels.
Attach the angle brackets to the fence post.  (This will use 2 nails per angle bracket, for a total of 8 nails.)
It is possible that one panel's rails will need to be slightly shortened.  (Perhaps the new fence post is not in exactly the same spot as the old one, or perhaps it is rotated slightly differently, or perhaps the fence post is slightly wider, or perhaps the extra thickness of the angle brackets does not quite fit between the rails and the post.)  If needed, use the saw to shorten the rails, so that they fit.
Attach the fence panels to the angle brackets.  (This will use another 2 nails per angle bracket, or another 8 nails.)
Reattach the two end fence slats to the fence panels.  (This will use another 4 nails.)


(Step 51 is a finishing touch.)


If the old fence post had a cap, remove it from the old fence post and put it on the new fence post.  If the cap is in poor shape, replace it.