Maneuvered it into the empty socket in the eyeball with the point of a toothpick, and pressed it in with the blunt end.
Honestly? If your ponies have their irises intact I wouldn't try replacing them. It's kind of a pain (the smallest diameter these come in is too big, so I had to grind this one down to make it fit), plus replacing the irises wouldn't fix the rest of the rust. I would try cleaning off the rust with Q-tips dipped in cola.