Finish the fold by lowering the shirt over the loose sleeve. If everything worked out, a folded shirt rectangle should be hanging down from your hands with a loose sleeve dangling at the bottom. Lower the shirt back down onto the table. The loose sleeve should contact the table first. Lay the rest of the shirt over the loose sleeve so it is not visible from the front.
If the loose sleeve isn't completely hidden after you lower the shirt, it's easy to tuck the sleeve under the shirt by hand.