For a pry-off type of watch back: to close the back, use a watch press. (The watch press tool and a set of dies costs about $15.)

Pick one of the dies that just fits the outer edge of the bezel for the front side, and pick another one that just fits the outer edge of the back casing. Attach the die for the back on the top (the lever of the watch press) and the die for the face on the bottom (the base of the watch press).

Place the watch face down. Arrange the seal (if any), and put the watch back gently into position.

Squeeze the tool slowly and firmly until you hear it click into place. Some watches do not click, so check the progress as you squeeze.