Now that you've reached and separated the thermal drain from the motherboard, it is time to clean heat exchange surfaces on both the drain and components.

The most recommended method is to use isopropyl alcohol and your microfiber cloth (to avoid lint sticking to electronics!) and gently wipe the thermal paste off the processor and graphics board (orange on this picture). They should look shiny and metallic once cleaned.

Similarly, you can take thermal patches off the thermal drain or electronic components (any other color than orange on this picture), and wipe corresponding surfaces using the same procedure as before. Don't throw your thermal pads right away however, you may want to measure them first!

/!\ That being said, sometimes some bits will simply not go off and in this case only, I suggest using an old smooth toothbrush to gently scrub the remaining traces (with isopropyl alcohol too). BE VERY CAREFULL for electronics can be extremely fragile! If something does not come off despite this method, do not persist. Just leave it.