Textures: The Texturing
I found a new way process textures that in my opinion works better.
It preserves the quality of normal maps without having to manually separate them from the rest of the mod.
The old way is below if for some reason you dislike this method.
Download these programs:

SSE NIF Optimizer - https://www.nexusmods.com/skyrimspecialedition/mods/4089
Octagon - https://www.nexusmods.com/skyrimspecialedition/mods/28773
Fallout 4 Texture Compressor - https://www.nexusmods.com/fallout4/mods/25691

-NOTE-
You do not need SSE NIF Optimizer and Octagon if ?Cathedral Assets Optimizer works for you.
Users have been running to problems regarding it.
If it works for you, just change your compression to "Uncompressed".
It should optimize both textures and meshes.
After this, follow steps 16-22.

1. Open up SSE NIF Optimizer
2. With it, open your meshes directory
3. Change the setting to "LE" on the lower right
4. Click "Optimize"
5. Close SSE NIF Optimizer
6. Open Octagon
7. In the general tab, open your textures directory
8. After that, go to the textures tab
9. Select "Add format/compression filter"
10. Click "Change texture format"
11. Change BC7_UNORM to B8G8R8A8_UNORM
12. Leave Octagon on "Don't change compression"
13. Click start and let it run
14. Keep clicking continue until the continue button disappears and only the close button remains
15. Close Octagon
16. Open Fallout 4 Texture Compressor
17. Select your texture directory
19. Check the "Ignore specular, normal, and glowmaps" box
20. Click "Start"
21. Let it run until completion
22. Close Fallout 4 Texture Compressor

Meshes & Textures

To port meshes and textures, you need only one program.
?Cathedral Assets Optimizer (NOT THE BETA) - https://www.nexusmods.com/skyrimspecialedition/mods/23316
Open Cathedral Assets Optimizer.
Create a new profile and use the TES5 profile as a base.
Name it whatever you want. I recommend naming it "SSE to TES5 Converter" so you know what it is.
Check "Show advanced settings".
Browse for your mod's data folder and select it.
Switch to the textures tab and check "Necessary Optimization".
Go down and change the output format to BC3_UNORM.
To be safe (theoretically it will work without doing this), go down to unwanted formats and click edit.
Check every BC format above BC3 (BC4, BC5, BC7).
Click okay.
Press "Run".
Meshes & textures should work after this.

-Mini Note-
Don't forget to mark "Always process faceparts" when processing things like face meshes and hair meshes.
Also, please do not compress normal maps (_msn or _n).
It dramatically reduces the quality.
Instead, use the uncompressed format (r8g8b8a8_unorm).

ESPs
ESPs are very tricky. If an ESP holds no masters besides core ESMs (skyrim.esm, update.esm, etc.), it is simple.
Open this link: https://www.facebook.com/Letertreduroliste/posts/898131110626413?__tn__=K-R
Download "Script de portage SSE vers LE pour xEdit"
Drag the .pas script file into the "Edit Scripts" directory in TES5Edit.
Open your ESP in the program where you installed the script.
Right click on your ESP in either TES5Edit and click "Apply Script".
Scroll down until you see "Convert SSE to LE."
Save your ESP.

If there are masters that cannot be ported (USSEP), follow this guide to remove them: https://www.youtube.com/watch?v=DyxDZT49CfM
After you fully remove it, add a master by loading your desired ESM with your edited ESP, right click ESP, and left click "Add Master" in TES5Edit (if applicable), for example USSEP --> USLEEP .
Here's a better way of changing ESP masters.
Get Wyre Bash and run it.
Select your desired ESP and go to the masters field.
If the masters field is too small, take your mouse and hover just below the save or cancel button.
You will then be able to expand the masters field.
Right click the SSE master and select "Change to..."
Select your desired LE master (ie. USSEP --> USLEEP)
Click the save button underneath the masters field.
Open your ESP in TES5Edit and check for errors.
If no errors appear, save your ESP.
If errors appear, search the afflicted item's FormID and resolve the error.
If an ESP comes up with unused data, don't touch it.
At best you are wasting your time and at worst you might break the ESP.
ESPs should work after this.