Swim backstroke, using your arms only; keep your legs still (use a pullbuoy or pullkick as an aid).
The pullbuoy will help you keep the right position in the water and reduce any resistance, increasing your power and hence your speed.
Placing the pullbuoy between your thighs will help your legs float better so you can focus on your head position. If you place it between your knees you will work on the alignment of your body. Finally, if you place it between your ankles, you will be able to focus on your core: so, try to relax your body as much as possible, imagine extending your head as far forwards as possible and relaxing your feet.
Tighten your stomach muscles and glutes to prevent your legs and feet from bobbing up and down.