Just align it and press down until you hear it snap. There are 4 snaps, each of which you will hear.
The scissor mechanism needs to be attached to the base, not the keycap, in order to be able to replace it. If you accidentally pulled off the scissor mechanism, remove it from the keycap and carefully replace it -- you might want to pull off the keycap of a square key to see how the scissor mechanism fits -- and then you can push down the keycap.
tooki