Instructions
Move the analog stick down and press "B" at the same time while in the air to take the form of a stone. Kirby will begin falling towards the ground.
Determine when you wish to break free of the stone. If a player is directly underneath you, it's best to try and hit him with the attack. Since Kirby falls quickly in stone form, it's unlikely a character will move if he's already directly underneath you. If the character moves or you miss with the attack, it's best to break free immediately.
Press the "B" button to transform out of stone form and back into standard form.