The layer should be as thin as possible but it should also be a solid colour. Say it's AS5, the layer should be a solid grey, and as smooth as possible. I find that a credit card makes it too thin, plus it ruins the card.
I usually put a tiny drop on then rub it with a finger, then slowly add more, until the whole chip is covered. Then I clean off the edges, as AS5 is conductive and I don't want it touching the motherboard.
See the link below but yes only a tiny pea size amount is required. You really want the thinnest possible film as TIM is only intended to fill the LOW SPOTS between the HSF and CPU heat spreader. Metal-to-metal is best. Most people use WAY too much TIM.