Use a coffee filter or a lint-free cloth and a little isopropyl alcohol (a.k.a. IPA, 90% concentration or greater) to clean the thermal paste residue off the thermal contact surface of your heat sink.

Instead of IPA, you may also use a dedicated cleaning agent, such as ArctiClean Thermal Material Remover.

Once the surface is clean, use a fresh piece of coffee filter or cloth and a little more IPA to remove any oils and prepare the surface.

Do not touch either the chip or the heatsink, or allow any dust or debris to get on them. Even a fingerprint can be a major obstacle to thermal transfer on a chip.

Allow the heat sink(s) to dry completely!