These caps press on the head of Socket Head Cap Screws. There are three designs; knurled, lobed, and winged for either a M3 or M4 screw. Use a hammer, c-clamp, vice, or arbor press to press the caps on the head of the screw. The press needs to be between .005-.01" to work properly. Print one out and test it before you print out a bunch of them. If the fit is not correct scale the .stl slightly in your slicer to correct it.