Open the game, click Mod Editor
In the Mods Editor window that will come up, click New Mod (directly under File, looks like a dogeared piece of paper)
Give your mod a title.
Double click your new mod in the list of mods. This will open up a Mod Editor window.
Give your mod a description and preview image (this can wait until you're ready to upload to Steam)
Click New, select Radio Station from the drop down list.
Give your radio station a name (used internally for the game to interact with it. Optionally, enter a comment about it.
Give your radio station a display name (what the player will see) and a folder (within your mod) where your music will be stored.
Select how many seconds of silence between tracks.
Optionally, give a playback function. The modding docs give one to shuffle the tracks and play them one at a time that you can use, until you're good enough with their version of LUA to write your own.
Stick your music files (docs say encoded with the OPUS codec) in the folder specified above. The mod you created will be in %APPDATA%/Roaming/Surviving Mars/Mods/<your mod name here>/<your music folder here>
Click the upload button and it gets sent to Steam, or zip it up to publish elsewhere. The mod you created will be in %APPDATA%/Roaming/Surviving Mars/Mods/
I hope that's clear enough.