During reassembly:

Use the flat end of a spudger to scrape off any large chunks of thermal paste from the bottom of the motherboard.

Use a pipette or syringe to apply a drop of isopropyl alcohol (>90%) to the thermal paste residue.

Wipe away all the residue with a coffee filter or lint-free cloth. Wait one minute to allow the alcohol to completely dry.

Apply a small bead of thermal paste to each of the four plus (+) signs that are lightly engraved in the frame.