Tombstone drill video
The Tombstone drill, named for the way a kickboard looks when you hold it vertically in the water, is designed to improve both technique and leg strength as the float provides extra resistance while you kick.
Kick with a float outstretched but held vertically in front of you to provide resistance (wear fins if you find this too difficult)
Keep the head down to keep the hips up until you need to pop up for air or use a snorkel.
Feel the big toes lightly brushing against each other to improve rhythm.
Aim for a straight leg upsweep, but it is ok to bend the knee slightly on the down sweep.
Avoid pointing the toes down towards the floor which will upset your streamlined position.