Use Thermal Material Remover to clean the chips and the heatsinks of the old thermal paste. Clean off both the CPU and northbridge chips. Wipe off the occluding surfaces with lint-free cloth.

Take care not to drop anything on the northbridge. It is not protected with a steel lid.

Open the latch on the processor cradle to clean around the CPU chip. Don't remove the CPU chip. That would be an unnecessary risk and there is nothing to clean underneath.