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!

I use this thermal paste. Worked well.

https://www.amazon.com/One-enjoy-TF8-Performance-Interface/dp/B07K442WXV

I use thermal right odyssey thermal pads. Also used gelid but they are a bit more rigid to my preference. Also kriticalpads also provides some good precut pads for certain GPU models.