Swim freestyle using your legs only, keeping your arms still and extended out in front of you with one hand on top of the other.
To make this drill easier you can rest your hands on a board. Simply raise your head when you need to breathe.
Concentrate on your feet: they should be well extended but relaxed. They should break the surface of the water but not be completely out of the water.
Do not bend your knees excessively, otherwise your feet will come too far out of the water and your forward thrust will be less efficient.
Tighten your glutes and make sure your leg kick is short and efficient.