Lay your baby on their back on a blanket and hold a noisy toy just where their feet naturally land.
Wait. Any kick that touches it makes a sound — react to that sound with delight so the connection lands.
Move the toy slightly so they have to reach a little differently each time. Three or four minutes is plenty.
Legs develop strength and control later than arms and get far less deliberate practice. A sound that only happens when the foot connects gives immediate feedback, which turns random kicking into aimed, repeated movement.