Carefully scrape off the thermal pads and the remains with an opening tool or a spudger.

Apply only little pressure to not damage any components on the motherboard in case you might slip with the tool. Also make sure you do not press the thermal pads in between small components as it is very cumbersome to get them out afterwards.

You might want to clean any remaining residue with a Q-tip and some isopropyl alcohol (>97%).

Be sure to only dab on the small components so the fibers of the Q-tip won't get caught.