Swim freestyle using your arms only: keep your legs still (use a pullbuoy or pullkick as an aid).
Make sure your body is not too stiff or tense, let your legs float naturally (an unnatural position is likely to give you cramp in your calves).
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 (imagine you have to keep your chest inside a rectangle) you will work on the alignment of your body, avoiding any excessive sideways movement and making your arm stroke more symmetrical. Finally, if you place it between your ankles, you will be able to focus on your core.