Forearms are easy. Just cut them out "correctly" and rivet them together. No button is needed here.
You should be able to slide your hand in and out of the forearms easily when done correctly. The gloves will come on last.

Cut away any pieces that may pinch you when you move your arms. I've found that the interior of the forearm is usually 9 inches long while the exterior is 14 inches long. It all depends on your taste.