The first set of eyes (for the Cthulu, from Jo-Ann's) went on fine. The owl, though.... the yellow cat eyes from Hobby Lobby proved rather more of a challenge. Try as I might, the backs would not go on to the eye posts. Finally, I had success with a hex wrench that fit around the post (without pushing the back of the washer closed). Put the eye against a hard surface and the back on the eye post with the wrench on top of it, and push down on the wrench.
Fair warning, though - don't do this on a surface you care about! (oops....)