Adding to jafaremraf's (excellent) response on correct rotation using clock positions and buoyancy. Often swimmers will empty lungs too quickly causing the body to sink. You need hold some air back without holding breath - very slow exahale. Empty lungs when rolling to breathe and quickly fill tank when nose/mouth breach surface.
Here's a video demonstrating what happens to the body exhaling too much too soon, as well as maintaining enough air in lungs for buoyancy: SwimVICE: Breathing and Buoyancy