Go to Nexus Mods (Skip to enable mods part if you are using steam workshop)
Download your chosen mod (usually as a .zip or .rar file).
Use WinRAR or 7-Zip to extract the mod.
Inside, there should be a folder (or several folders), usually named after the mod.
Go to your Bannerlord installation directory.

C:\Program Files (x86)\Steam\steamapps\common\Mount & Blade II Bannerlord\Modules

Copy the extracted mod folder(s) into the Modules folder.
Enable Mod in Launcher
Start Bannerlord.
In the game launcher, go to the Mods tab.
Tick/activate the mods you want to use.
Make sure they’re in the correct load order (check mod page if unsure).
Click Play. Enjoy your mods!