Line up the parts and check again that you have everything clean and ready.

The northbridge heatsink is now shiny clean. Notice that the copper occluding surface of the CPU heatsink has also been cleaned of old paste. The PCB has been cleaned thoroughly on both sides.

At the lower right you can see a pair of tweezers. These are useful for picking up recalcitrant blobs of dust and occasional cotton threads that a cotton bud may leave behind. Take ultra-care when using metal tools on the board, to avoid scratching components.