Kicking mistakes in freestyle

There are three major mistakes many swimmers make in the flutter kick: kicking too high, kicking too low, and bending the legs too much on the upbeat.

1. Swimmers are kicking too high when the entire foot and part of the leg come out of the water. Excessive upward leg movements may push the hips down. The feet should reach the surface during the upbeat of the flutter kick, but the entire foot should not come out of the water.
2. When swimmers kick too deep, they increase frontal surface area unnecessarily, which increases form drag. They may also push water forward during completion of the downbeat. The feet should be only slightly below the chest when they complete each downbeat.
3. Swimmers who bend the leg during the upbeat push the water forward with the underside of the lower leg. This, in turn, submerges the hips and decelerates forward speed. The upbeat should be made with a straight leg and swimmers should allow the lower leg to flex until it starts the next downbeat. Above all, they should not push the leg against the pressure of the water during the upbeat and the first part of the downbeat. Rather, they should allow the pressure of the water above the leg to maintain it in an extended position during the upbeat. Then they should allow the pressure of the water below the lower leg to flex it during the first part of the downbeat. The only time swimmers should push against the water is during the final portion of the downbeat, when they extend the lower leg vigorously at the knee.

A very good drill that will definetely help you improve your flutter kick as well as it will teach you diagonal kicks and body rotation is side kicking drill, and any drill that contains side kicking like 6-kick switch drills (see here).