Games developed with high level modern languages rely essentially on files. Wether it's for resources (textures, models...) or text information for the engine (configurations, scripts...). Thankfully we are way past hardcoding using registry values.

Some developers are modder friendly, some aren't.

Uncommon or proprietary archive types might have published unpacking tools.
Lucky for us Nem has developed quite a set of tools for quite sometime now for GoldSource and Source engines.

We'll take a look at GCFScape[nemstools.github.io].

For our purpose, GCFScape will help opening VPK archives.

As an example you can install a mod for Left 4 Dead 2, and explore it's contents. A good way to get familiar with the specific workings of each file in the game. You'll find said mods usually in :

C:\Steam\steamapps\common\Left 4 Dead 2\left4dead2\addons\workshop

It's always preferable to make a copy of the file you're working on to avoid corruption of the original data. It serves you as a backup.

Launch GCFScape and open your .vpk file.

Folder/File structure is very important, as a beginner, it's preferable to place each modified file in the original's place. Customizing paths can be limited by the engine itself, or requires configuration.

Editing software varies of course depending on the files you extract. You can always Google the "extension" with "source engine sdk" for example. That will give you insight on what you can do with said files.

When you're done, make sure you consider crediting and asking permission for others work if you are using part of it. Only publishing is left.

Valve developed the workshop for browsing and publishing, a tool for packing the modified files and a GUI for uploading your mods.

Packing your files is as simple as dragging your mod folder on vpk.exe.
For our example vpk.exe is located in :

C:\Steam\steamapps\common\Left 4 Dead 2\bin\vpk.exe

A shortcut to said program can be handy if placed on the desktop.
If there are no hiccups it will generate a vpk file with your mod folder name.

For publishing said archive we can use for our example :
Steam Library > tools > Left 4 Dead 2 : Authoring Tools

Launch said menu, click on Workshop Manager and fill in the appropriate fields.
That's it.

Please rate this guide if you find it useful, it helps making it visible to other users.

If you have related questions or suggestions, feel free to leave them in the comments section and I would try to answer them.

Thank you.