First, find the mod that you want on the steam workshop and download it. Each mod will have it's own ID in the URL of the page. Using Gespent's Infinite Carryweight as an example, which can be found here: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3328065171

In the URL of said mod, the number at the end I.E. 3328065171 is the important bit.

Now that we have our mod ID, we need to find our world. The save for your world can be found at C:\Users\you\AppData\Roaming\WizardsOfTheCode\Soulash2\saves\yourworldname

There is a file there called general.json. We will need to modify it with notepad or something similar.
The contents of the file will look something like this:

{"day":75,"game_version":"0.8.8","last_update":"2024-09-09 17:36:24","player_name":"Maddox Limbaum","required_mods":["core_2"],"seed":"DltRj","world_name":"a","year":26}

The bracketed section after "required_mods": will need to be edited with that mod ID we found earlier. Using Gespent's Infinite Carryweight mod as an example, we must change "required_mods":["core_2"] to "required_mods":["core_2","3328065171"] and then save. Any further mods will require another comma and their own ID inserted between quotes. If all goes well, this will now make your current unmodded game into a modded game with all of the features that your inserted mod will provide.

It probably only works on simplistic mods that give recipes and skilltrees and such, don't expect a mod that makes changes to worldgen or inserts a race into your game to actually work. I've also found that you can remove mods by removing the information after "core_2" behind the square bracket as well but that probably will also cause major issues for anything but the simplest of mods.

Also if you're looking to add a non-workshop mod, you'll probably need to find how your mod is defined by creating a new world using only that mod and see what said mod is defined as next to core_2 in general.json