Extend one arm in front of you, palm up, and use your other hand to gently pull your fingers back toward you until you feel a stretch along the underside of your forearm. Hold there, then turn your palm down and pull your fingers toward your body the other way to feel the stretch shift to the top of your forearm instead. Keep the elbow of the arm you're stretching straight but not locked hard, and pull from your wrist rather than yanking your whole arm back. This covers both directions in one go — forearm tightness from gripping usually shows up on one side or the other depending on the day, so check both rather than assuming.