It is preferable to have a crystal press to re-attach the crystal retaining ring. Many people do it by hand, but what can I say- there's a lot of things you can do by hand, but it is better to use the right tool. Using a crystal press with the right size dies allows even force (not "evil force") to be applied to the retaining ring so that it snaps into place. Repeatedly doing it by hand can warp the ring and/or not ensure a proper seating of the crystal and ring.
This is the same principle with snap-case backs. It is preferable there too to use a case closing tool (which doubles as the crystal press, with different dies sometimes needed), as the even forces used will prolong the life of the case back, ensure proper seating of the gasket and case back, and avoid warping the case or case back.