If you are right-handed, start by swinging it from your dominant side.
Spread your feet just lightly wider than shoulder-width apart. Bend your knees slightly. You want a strong, stable base. Your feet should never leave the ground or shuffle or move.
Place your left hand near the bottom of the handle. Place your right hand at the top of the handle, just under the rubber head.
Using the strength of your right arm, swing the head of the hammer in an arc – up, back, then forward.
As the hammer starts its upward travel, slide your right hand down the smooth finish of the handle until it just touches your left hand. At this point the hammer should be at its maximum height.
Explosively drive the hammer down. Use your whole body. This is not a strength move, it’s a power move. When done correctly, your arms will be fully extended at the instant of impact. Every muscle in your body will be tensed and assisting.
Stare at one point on the ground when you do this. I find it most helpful to aim at a mark on the ground.