This is a brief and simple tutorial on custom music for SBM.

First things first, prepare some music that you'd like to hear in-game. Do try to cover all three active combat types: aliens, soldiers and bosses, along with ambient. Otherwise end users will have to resort to installing other packs to fill in the missing spots.

Next up, do any trimming and cropping you need and convert the result to .ogg. It is preferable that the track you're making gradually fades in and out. Avoid abrupt starts and cuts, Audacity is a good tool for that.

EDIT: downline made a tool that can automatically convert your tracks to .ogg, along with fading then in and out, get it here: https://github.com/johnpaster/sbmconverter
you should use audacity though: https://www.audacityteam.org/download/

Finally, get the total playtime of the track you just made, in seconds. Add this value to the end of the filename, starting with an underscore.
It should look like this: TRACKNAME_999.ogg, _999 being the playtime.

EDIT: If you track is shorter than 100 seconds, make sure to use _0XX instead of _XX

Now download the template: https://mega nz/file/cFkVyaxT#zzWrpcwHdsdqkuT1gODmG03eKwA2SH-3gMx6X6yvTfM

Unpack it into your "Garry's Mod/garrysmod/addons" folder and start filling up the track types. Remember that you need to preserve the original folder structure the file comes with, including the initial template folder. You can rename it later, just don't forget that you need it. There are no minimal requirements, just do your best to have at least one track in each folder.
After you finish spreading out the tracks, clean up all the dummy files.
Once you're done with all that, give whatever you just created a bit of playtesting. Debug commands might help you with that.

If it works correctly, feel free to upload it to the workshop, using the template as your base folder. A universal frame for whatever image you picked as a logo is included in the archive, do use it to keep things noticable and recognizable. There's also a template for your addon name and description. Don't forget to add the base mod as a required item.

You can also convert old DYNAMO packs very easily to SBM.