I have no doubt that this is not how this forum is supposed to be used, but I have no idea otherwise. I’d like to download some mods for Black Mesa, (Xen remakes, mostly) but I don’t know how. Would some kind person post a simple tutorial on downloading mods?
Certainly.
Most mods will have a specific place you can download them from, and they aren’t all that hard to find either. If you have a certain mod in mind that you want, it doesn’t take much to find out where you can download it from (a quick search at most). From my experience you’re find plenty of links to the various mods in the various Sector C threads, oftentimes in the very first post of the thread.
While most mods have instructions on how to install, they follow one similar detail: where exactly to place them. BMS resides in a special place within the steam directory. It can usually be found in the steam/steamapps/sourcemods directory. Whenever you download a mod - whether it be a Xen remake, a texture mod, one of Text’s Uncut mods, sound mods, or what have you, you’ll usually extract the contents to the BMS folder in your sourcemods directory (as most mods are given as zip files).
That’s pretty much all there is too it, it’s pretty plug and play at this point.
Great! Does it matter which subfolder I put the files in?
Yes. Black Mesa (or just about any other game that uses this file system) has to have everything in a certain place. Thankfully it’s really consistent. Just pasting the files in the BMS folder is usually enough to get the job done. All you have to do is make sure to say yes to merging folders.
When in doubt, check where you are. If you find yourself in cfg or maps or sounds or any other folder, you’ve gone too far!
Some mods will have installers, and you can just download those and execute them and they’ll install and rewrite files on their own. Others will be a zip file. In that case, just extract the zip into the root BMS folder.
Buuuuuuut what if you didn’t know how to extract zip files to other folders/you couldn’t merge folders? And before you ask, it works fine for me. 
(Well, most of the time. Sometimes the folders wont merge at all and I have to manually transfer every single file, which is a pain :fffuuu 