Use isopropyl alcohol (wipes or liquid on a q-tip, 90% or higher) to wipe off thermal paste from the CPU die AND THE COOLER. If using isopropyl alcohol under 90%, make sure the wipe or q-tip is damp, and use some scrubbing force with a fingernail or toothpick (thermal paste gets on skin and clothes and is hard to remove, use gloves).

Make sure the CPU die and cooler are completely clean of thermal paste, and dry. If not dry, use a regular paper towel to dry off.