Like with the heat sink, use a coffee filter or a lint-free cloth and a couple drops of >90% isopropyl alcohol to clean the remaining thermal paste residue off the processor surface.

You can alternatively use a dedicated cleaning agent such as Arctic's ArctiClean or isopropyl alcohol swaps or wipes for the processor as well.

Once the surface is mostly clean, use another, fresh piece of coffee filter or lint-free cloth with more IPA to further remove any oils or extra residue.

Avoid touching the processor during this procedure. Small dust particles or oils from fingerprints can reduce the effectiveness of the thermal paste application or otherwise harm your chip.

Allow the processor to dry completely.