this is basically how i do it (please bear in mind i am rubbish at explaining things)
(on your computer) add mp3 files to spotify
in Settings it gives you options to add your entire music library to spotify, or "add a source" to choose specific folders. example...
add tracks to a playlist
the songs should now be in Local Files, you can add them to a playlist from there. example...
(on your phone) select the playlist and download
i think the desktop spotify client has to be open at the same time.
and that is how to do it
(welcome to DiS by the way)