This one is easy.
Go to Audiosurf 2's folder in Steam's files (you can go to the game's properties from your steam library, go to local files, and hit "Browse Local Files" to do this). Go into either 'skins' or 'mods' (doesn't matter which one you pick) and make a new folder in it. Name it whatever you want. Next, copy all the contents from another folder (I recommend using one of the default ones rather than cloning someone's custom mod) and paste it into yours. Rename the [mod/skin].lua name to [yourfoldername].lua.
Boot up the game and go into either the skin or mods selection and find your folder name. There should be an arrow on the thumbnail of it. Click it and it will upload it to the workshop. That should give you the achievement. If you don't want your mod in the workshop, you can delete it from its workshop page (you'll still have the achievement if you do this).