Download custom maps here: https://tsg.mod.io/

Drag and drop the folder from the archive into C:\Program Files (x86)\Steam\steamapps\common\thesnowboardgame\CustomLevels\

Don't rename the level folder and make sure there are no sub folders. All the map files should be like this :
CustomLevels \ [level folder] \ [level files]

EDIT (january 2022) : Do not use the experimental version of the game anymore

Keep in mind that custom maps may take a while to load. Sometimes the game can crash if you change custom maps several times. You will have to restart the game.

Multiplayer is working in custom maps, but you may see people from other custom maps floating in the sky :)

You need to redownload them and replace the old map files in your map folder.