Step 1: Gather Materials
3.) Screenguard card, or credit card
4.) Magic scotch tape
5.) Permanent marker (should be fine tip)
6.) Good pair of scissors
7.) Windex and paper towels
Step 2: Prepping the Screenguard and Cutting
Try to position it so that you can re-use the rest of the screenguard on your watch in the future. The PSP screenguards will be able to cover a standard watch face three times.
Using your marker, carefully draw around the edge of the watch face, staying as close to the glass as possible, then remove the screenguard and tape from the watch. Using your scissors, cut the circle out. This part is crucial: WHEN CUTTING, CUT TO THE INSIDE OF THE LINE YOU MADE. If you cut around the outside, it will be too big, and overlap the glass, and you will have to trim. Cutting around the inside of the line should give you exactly the right size.
Place the circle over the watch face to verify that the size is correct. You may still have to trim a bit, but not much. Do this carefully, and a little at a time. Too much, and you have to start over.
EDIT: Alternatively, you could also measure the face area of the watch, and then use a compass with a marker to draw the circle. You will want to be careful if you do it this way, because if you press too hard on the pivot, you could accidentally place an indent or hole in the center of your usable screenguard. I would suggest drawing the circle on paper with the compass and a pencil, then using the magic tape, attach the uncut screenguard to the paper, and trace over the drawn circle. Remember, when you measure across the watch face, divide that number in half before you draw the circle. Thanks to gmjhowe, for the suggestion.
Step 3: Prep Watch and Apply Cover
Carefully peel the screenguard apart halfway. With the backing still partially on, align the edge of the screenguard with the watch face, then pressing down on the applied portion, remove the rest of the backing. Use the card to smooth out the bubbles, if any, then you are done.