Grasp with both hands around their wrist just above the hand and squeeze firmly (but make sure it isn’t too hard). Ask them to tell you if they want you to squeeze harder or more lightly. Continue squeezing and opening your hands as you slowly work your way up their forearm to their elbow. Then work from their elbow back down to the wrist. Repeat this ~10 times. Ask them to relax their arm and shoulders. Gently sway their arm every now and then to make sure it isn’t stiff. If it is stiff, ask them to relax their arms again.
Keep working up and down their forearm longer if they say it feels really good.