Open Spotify on your computer
Click your profile picture or the arrow next to it
Go to Settings
Find Local Files
Turn on Show Local Files
Click Add a source
Select the folder containing your music files
Make sure the files are in a supported format
Wait for Spotify to scan the folder
Open Your Library
Go to Local Files
Add the songs to a playlist if you want them on your mobile device
Make sure your computer and phone are on the same Spotify account
Download the playlist on your phone if local file syncing is available