It's quite simple to use, simply browse to the "mods" menu in SnowRunner, and follow the instructions given on the screen.
You can then browse the mods available, and when you find one you like, press the "subscribe" button to add it to your mod library. Once downloaded, you can activate it from the mod menu and purchase/use it within the game.
If you're still stuck, you can ask on the SnowRunner Discord Server here; https://discord.com/invite/snowrunner