I tend to be very accident prone, and because of this, every watch I've ever owned has tended to get scratched up and scuffed pretty bad. So when I got a much sought after binary watch for my birthday, I knew I needed to either come up with a way to protect the face better, or not wear it very much. And a watch like this begs to be showed off! So, what I did, was make a screenguard for my watch. Today, I replaced the screenguard (they do need to be replaced periodically), and I thought I would share my idea with you.

Step 1: Gather Materials

For this, you will need:

1.) Watch
2.) Screenguard
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

Fold a curl of magic tape (make it doublesided) and place it on the watch face. Then position the screenguard over the watch, and secure it to the tape. I used a PSP screenguard, because I have a PSP, and found a bunch on sale once, so I have plenty to spare.
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

Spray a bit of Windex onto a paper towel and clean the watch face. Use a dry portion of the towel to remove excess moisture.
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.