Remove the motherboard from the phone.

During reassembly:

Clean off the old thermal paste from the midframe and motherboard using isopropyl alcohol and a microfiber cloth. Apply new thermal paste before reassembly.

Ensure that all of the cables that connect to the motherboard (interconnect, battery, antenna, display, and button assembly) are out of they way so they do not get stuck under the motherboard when it's reinserted.