You should get two bars in a pack. Break one in half. Work the piece with your hands until it is soft and pliable. Stick the other piece in hot/warm water. Mould the piece in your hand so it's fairly flat.
Spray lube onto the car and gently rub the clay across the surface. It should skim across smoothly. If it catches, there's not enough lube. Lack of lube means friction and hence paintwork marring.
Every half panel or so, inspect the clay surface. If it's dirty, fold in half, reflatten and continue. If you end up with a dirty surface every time you fold, chuck it away. If you drop it, chuck it away. If it becomes hard or brittle (mainly in cold weather) stick it in the hot water and use the other piece.
Go slowly - you'll use loads of lube and clay on a previously unclayed car. I prefer to use Born Slippy with Bilt Hamber soft autoclay.
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