Plank on Feet to Shoulderstand on Hands is a good exercise to learn to keep a tight and straight body during a transition for the flyer and for transfering the flyers weight for the base.
Walk/wiggle your feet to the flyer's lower thighs. Flex your feet and bend your legs and reach for the shoulders of the flyer. Place your hands on the shoulders and shoulder blades of the flyer and make cup hands. Extend the legs slowly and by pointing your toes guide the flyer to the shoulderstand position. Lock your ellbow and let your shoulders sink into the floor.
Keep your body straight or slightly hollow. Stay tight (engage the core and your legs). Place your hands on the forearms of the base in the front, fingers point down.
Push your hands down on the forearms of the base to come to the shoulderstand position and to keep the balance.
The most likely crashes are that the flyer is falling backwards away from base and the flyer collapsing onto base’s face or ribs.
Spot this from knees to the side of the base's head, with my outside arm (arm nearest the head) on the far shoulder. You can then directly influence where the shoulders are in space, as well as provide immediate lifting force to the shoulders as soon as there is a fall. The inside arm (nearest base's hip) can assist the flyer's hips if needed, or if they're going over. Do not spot this standing! It is much harder to adequately impede a quickly falling flyer. (spotting tips courtesy of Chris Cox)