Try lacing a block between your hands. Keeping your elbows shoulder-distance apart, press the palms, especially the knuckle of the index fingers, into the block. Keep the bottoms of your wrists rooted and press your forearms down. Extend your fingers forward along the edges of the block. Cut the forearms down through the floor. Engage your outer upper arms. Keep the inner elbows rooted and strongly lift the shoulders up away from your elbows.
See also Upside-Down Prep: Dolphin Pose