as chris point out having a thermal pad or copper shim between the internal copper plate and the outer bottom shell should do the job. You should proceed with the advance and by extension more dangerous stuff only if you have to.
If you are still planning on going through with the mod then this is a good item to get -https://www.ebay.com/itm/30x-Heatsink-Copper-Shim-15mmx15mm-for-Notebook-Laptop-GPU-CPU-6-Sizes-Durable-/331167307336?hash=item4d1b1c1648
ebay and aliexpress should have most of the other item you are looking for, the good stuff like arctic silver helps but not by much. For the cleaning agent isopropyl rubbing alcohol will work just fine.
Anyway there is no correct way to do it per se, watch through a few of the thermal mod videos and try to figure out the thought process rather than the individual steps.