So as mentioned, I've done many 3090 FE's. It's really not that tough; you just need the right toolkit and materials.
Here's the breakdown of the pad cutouts: https://i.imgur.com/KzLhN62.png
Here's the pads: https://www.amazon.com/gp/product/B06WWDT6RC (you'll need 4)
For the Thermal paste, I recommend TF8: https://www.amazon.com/gp/product/B07K442WXV
Make sure you SLATHER that thermal paste on; it's not like a CPU (which requires a thin layer). The GPU requires a lot more. Here's an example:
https://i.imgur.com/0Nc5FZc.jpg
https://i.imgur.com/5IP717e.jpg
If you don't have a kit, I recommend this one: https://www.amazon.com/gp/product/B07ZNHWHV5
Oh! And it's best to draw the cutout patterns on the gelid pads plastic cover with a ruler and perm fine tip marker - 10x easier to properly cut them before placing them.
Let me know if you have any additional questions!
https://www.amazon.com/nkomax-Thermalright-Conductive-Resistance-Temperature/dp/B08CGVZ4YG/
That's what I used on my backplate. I've seen another use it for both sides and it worked well for him. I'd just use this on both if I ever did it again for convenience
https://www.amazon.com/Gelid-Solutions-GP-Extreme-TP-GP01-C-Thermal/dp/B06WWDT6RC/
This is the brand I used for front side. But it doesn't seem to have amazon prime shipping. It's the 1.5mm version (I used 3mm but it was too thick)
As for thermal paste on GPU, I used Thermalright TF8. I can't link it because the one I bought went out of stock. They still have it on amazon, but they have the smaller version and I've seen some reviews saying it arrived dry. That's why I shelled out extra for the bigger version because no one had QC issues with them
