The ones I do are mostly heavy isometrics. I do weighted dead hangs for the inside forearms and isometric reverse grip barbell curls for the outside forearms. For the curls I'll hold at about 90 degrees. For both I shoot for more than 20 second holds with as much weight as I can. The specific exercises aren't as important as doing something that creates a lot of tension on the tendons you want to work, sustaining the tension, and being safe. The last thing you want is to exacerbate the problem.