1) Find the data folder in the game installation directory.
2) Find data.pak there.
3) Open it using WinRAR.
4) Understand its structure. This comes with experience ;)
5) Find the files you want to edit. For small balance mods, it is usually enough to open GameMechanics\Creature\Creatures[faction name][monster name].xdb.
6) Open it with a suitable program (xdb -- notepad, dds -- some image editor, there are usually no other file types).
7) Understand its structure. Usually, a little knowledge of English is enough for this.
8) Edit. Save the modified file somewhere on your desktop or wherever it is convenient for you.
9) Create a group of nested folders so that their names repeat the names of the folders in data.pak. Place our file inside. In our case, we will have a GameMechanics folder on the desktop, inside it a Creature folder, inside it a Creatures folder, and so on.
10) Archive all this into a zip archive.
11) Rename the zip to h5u.
12) Place it in UserMODs.
13) The mod is ready and working.