how do i back up my files from spotify onto icloud

You can't. Only music purchased from iTunes is available through iCloud, unless you subscribe to iTunes Match. And even with iTunes Match it has to be music you own (without DRM) imported to your iTunes library and music purchased from iTunes in order to be available through iCloud. Music from sources such as Spodify is not supported.

Actually not only you can, but it is the case by default: go to your settings iCloud STorage -> Backup options -> you should see the list of apps that are backed up. Spotify should be there.