Apply a small amount (official guide says 0.2g) of thermal paste on each - CPU and GPU cooling contacts. CPU is longer, so apply a line, while applying a dot fits the GPU contact point better.

When putting the the heatsink back in, tighten the screws in the order written on the heatsink - from 1 to 6. You cannot overtigthen, as these screws simply stop at their designated torque level.