I cut my own hair, and have done so for many many years. My Wahl clippers weren't cutting very good, so I googled how to sharpen them.
Took the blades off, sharpened on a stone, cleaned, oiled, put back together and good to go. The 2nd set that I had given to my son had stopped working altogether(made noise but blades wouldn't move). We cleaned and sharpened that one too, good as new.
A tip I found helpful is to color the surface being sharpened with a sharpie. Sharpen until all the black is gone, no spot missed.