If you have two electrodes I would do the muscle belly of the wrist extensors…. So if you lying down with your palm facing down it would be about three finger widths down from your elbow crease and two finger widths in from the outer edge of the arm. The other pad I would put three finger widths above the elbow joint line (crease).
..make sure you are also icing every hour for 15 minutes to help decrease the inflammation. Once the inflammation has resolved make sure to follow this event by stretching and strengthening the wrist flexors and extensors to help avoid the problem in the future. If you continue to have problems get a referral to see a physical therapist.