Hi there,
1.) Use a soft material (eg. rubber pad) as the base.
2.) Place on watch on top facing you.
3.) Put some toothpaste on the watch crystal
4.) Get a wet towel and press on the toothpaste and start rotate your finger in circular motion
5.) Wash off the toothpaste and you'll see some improvement.
6.) If unsatisfy, repeat the steps again until the scratches disappear.
Hope this helps.