Use thermal paste of your liking and apply a thin layer (I almost used too much).

The thermal pad (red circle) should not be replaced with thermal paste. If it seems dried out you have to use one of the same size.

Reattach the heatsink very carefully and tighten the screws in the given order. Also don't forget to connect the fan.

If your fan seems to be clogged with dust you should clean it while it's removed.