The drill #2 in my book: Simple Swims:Practices for balanced, stable swimming.
The most important drill to prepare you for swimming freestyle.
Gently push off from the floor of the pool, float face down in the water, exhale slowly through the nose or mouth, and use a gentle flutter kick. The arms are extended, sloping down. Let the nose point down to the bottom of the pool; press the hips up using pelvic extension as the chest presses down using spinal extension. The practice is to create a horizontal body position, from the top of the head to the tailbone.