First of all, you're going to want to go to the Blade And Sorcery Nexus Mod Page [www.nexusmods.com]
to download some mods. After clicking on one, press "Manual" on the mod's page to download it.
Now we'll begin to move the mods to the streamingassets folder, explained on the next section.

The StreamingAssets folder (where you'll be putting your mods) is located in
(Your Steam Folder)\steamapps\common\Blade & Sorcery\BladeAndSorcery_Data\StreamingAssets
In there, you'll find nothing but a folder called "Default".
Do NOT delete that folder. Doing that'll make BaS crash.

Now, your mods should come in as zipped.
Before you begin transfering them, you'll need to open them and extract the folder inside them.
To install the mods, place the folder inside StreamingAssets (NOT inside Default).

That's it!
Your mods have been successfully installed.

After the 8.4 beta, mods have to be put into a different folder, though it's in a very similar location. It's called the "Mods" folder, located in:

(Your Steam Folder)\steamapps\common\Blade & Sorcery\BladeAndSorcery_Data\StreamingAssets\Mods

Now, your mods should come in as zipped.
Before you begin transfering them, you'll need to open them and extract the folder inside them.
To install the mods, place the folder inside Mods

That's it!
Your mods have been successfully installed.

(Make sure to check your mod's version! 8.3 mods don't work with 9.3 and viceversa)