Okay first of all have your Spotify Desktop App open, >> now drag the files from your computer to the App and put them on 'Local Files' left bar of the app. >> After you've transfered everything there create a new Playlist name it. >> Get all the tracks you want to be on your iPhone from Local Files (select multiple by holding Ctrl on keyboard while clicking tracks) >> Be sure your phone is connected to the same Wi-Fi network as your computer and now open Spotify on your iPhone >> In playlists search for the new one you've created, you should see the tracks in gray, now click "Make Available Offline" on your phone (the desktop Spotify App must be open too) >> You should see your playlist will start downloading. (Do not lock your phone or exit the app as it may interrupt the download.) You are set!