Installation and required tools

Installation:

Unzip the file anywhere, as long as you have rights on the folder. Don't unzip it in a system protected folder.

Run Octagon.exe

Assure you're running the program under administrator rights if you don't want any surprises.

Required tools:

This program use .NET framework from Microsoft version 4.5.2
- If you are a Windows 10 user, it's already on your computer.
- Windows 8 and 7 users should be ok too and got it from Windows update some time ago
- If you don't, you can grab it here

This program needs texdiag and texconv program to operate changes on your textures.
- They are bundled in the zip.

Optional tools:

This program needs Bsarch to operate on bethesda archives. You don't need it if your textures are loose files only.
- You can grab it here

You can use GMIC to apply filter to your textures:
- You can grab it here
Go to G'MIC for Windows - Other interfaces and grab Command-line interface (CLI)
For more info on GMIC (Open-Source Framework for Image Processing)
You can see here what GMIC can do.
GMIC is really time consuming. Try it on some texture before applying  it to a larger scale, cause it can take the night. Or two.

User who want to upscale their textures:
You'll need a custom tool, callable in command line for processing your upscaling. I tested with Waifu2x so this tool work, but there is a ton of upscaling ai tools so make you pick.