I used Arctic MX-4 for these step for its thermal properties, and it was hanging around in my desk. You can use any thermal paste that you would like, just use the same process.

Drop a pea sized drop on each die. As long as your paste is not electrically conductive, do not worry if you put a little too much on.

You'll know if you have put the right amount of thermal paste if your temperatures are lower/you are not thermal throttling anymore.