Note that this guide assumes you have a basic understanding of how to navigate your computer's file directories. Also, it bears repeating; if you modify someone else's work, always ask permission from the original author if you want to reuploading your modified version to the Workshop, or you can get into trouble.

First and foremost, you'll need to download GCFScape (it's basically WinRar for .vpk files, which you can download from https://gamebanana.com/tools/26 )

So every Black Mesa Workshop mod you subscribe to gets downloaded to a specific folder. The directory should be something like this:
C:\Program Files (x86)\Steam\steamapps\workshop\content\362890\"MOD ID"
To find the specific mod ID that you're looking for, it's the string of ten digits that the Workshop page's web directory address ends with. The fastest way to find it is to open the mod you want to edit's Workshop page IN A NEW WINDOW, then copy the last the last ten digits of the webpage listed in green at the top-left of the screen. If you don't open it in a new window, Steam will hide this green address (for some reason). You can also just right-click the page and click "Copy Page URL".

CLICK TO ENLARGE

Once inside the desired mod directory, just open the FILENAME.vpk file with GCFScape, right-click on the "root" folder, click extract, and pick a directory to extract it to.

CLICK TO ENLARGE

Then move that "root" folder into your Black Mesa's "custom" directory, which should be at:
C:\Program Files (x86)\Steam\steamapps\common\Black Mesa\bms\custom

Make whatever tweaks you want to the files inside. Also, make sure to unsubscribe from the original mod just in case, so it can't overwrite your version. Hope this helps. :)

Note that turning a folder back into a .vpk file falls outside the scope of this guide, but mods will work even in folder-form if the formatting is correct.