How do I get invisible edges on my spackle jobs?
Remove high spots by hand sanding the entire wall with a flat, rigid 1/2-sheet sanding block and 80 grit paper.  It's ok to remove too much, but stop wherever you start to see the sheet rock facing paper or joint tape. Ideally, 100% of the wall will have been sanded.
For the areas (if any) where the paper shows before all the ridges (as in your photo) are gone, switch to either a 1/3-sheet or 1/4-sheet flat and rigid sanding block with 100 grit and sand broadly over the areas surrounding the show-throughs.
Remove the dust by wiping down the wall with a damp cloth/sponge kept clean by frequent rinsing in a large bucket.
Apply a thin coat of spackle using a 10" or larger metal blade. Apply less than required.
When dry, shave off any specs etc using the 10" blade over the entire wall.
Use a damp 4"x6"x2" sponge to 'sand' the new spackle,  working the sponge from the middle of a patch of spackle outwards towards the edge and beyond.
Apply a second thin coat of spackle and repeat the shave/sponge steps.
Inspect for flaws using a bright flashlight or trouble-light held against the wall with the room lights turned off.
Touch up as necessary.