What is the process for power washing and staining a deck?
The first step is to clean the deck.  A very simple and inexpensive method is to wet the deck, spray or scrub on a mixture of 1 cup TSP, 1/2 gal household bleach, and 2 gal water. Scrub it with a course, stiff broom. Then either rinse with a hose or power wash off before it dries completely on the surface. this works as good it not better than expensive deck cleaners. Note: When using a power washer, use a wide spray nozzle. Be careful not to be too aggressive with a power washer as it can damage the wood and leave divots.

Second, let the deck dry completely. This may take a day or two depending on weather conditions.

Third, apply deck stain.  You can use any good oil or modified oil based deck stain. You can choose from transparent, semi transparent or solid.  The most popular for older pressure treated decks is semi or solid.  For cedar or redwoods, transparent or semi is favored. Usually one coat is enough, but on very porous or softer woods, two coats may be required for a good even finish.

Application hint: apply stain with a 3/8" nap roller on a pole. Do three or four boards wide, then over-brush the area with a fairly stiff bristle brush to work the stain into the grain of the wood. You could also use a 1 or 2 gal pump sprayer to get the stain on fast, then over-brush.  A lot faster and easier than using a brush only.

You're done. There is no need to put a sealer over good grade stains. Never put a sealer on before you stain, as it will block the stain from penetrating and adhering to the wood. 

Wait 24 hours before using your deck after staining.  Don't stain if there is rain or temps under 50F predicted within 12 to 24 hours.