How much thermal paste should apply to the CPU?

You should use the absolute minimum - just enough to ensure that any micro-pits in the heatsink and CPU top are filled. I spread it like (very thin) butter using the end of a plastic ruler and then give the heatsink a gentle slide a few mm in every direction once it is in place and before I clamp it down.

Protip 1: Avoid using toothpaste!

Protip 2: Regardless of how careful you are, the damn stuff gets everywhere and is not water soluble so don't wear your favourite top or trousers/pants when using the stuff and don't do the work on that valuable, antique black lace table covering.

That depends on the paste you are using. I use a non-conducting colloidal silver paste and apply just a small, grain of rice sized, dab in the middle and spread it out with a plastic bag protected finger. The surface of the die has to be immaculate as in cleaned with acetone first.