Start with breathing in and make sure your lungs are full of air. Without your lungs full of air it's harder to float in water.
Hold your breathe to keep floating. Only exhale half amount of air from your lungs to stay afloat in water.
Keep fluttering your feet gently to keep your legs floating on the surface.
Don't stop kicking or your legs will start sinking.
Flutter your arms gently waving goodbye to your feet. Keep moving your arms and legs to keep moving in water.
Don't raise your hands too high above the surface of the water else you will start sinking down.
Keep your chin up with when floating on your back. Head being up like this is like front quadrant body position. It is also relaxing and will help you float longer on your back.
Don't lift your head too high in the air. Doing this will sink your lower body and your legs will pull you down in the water.