Open Spotify and start playing the song
Tap the Now Playing bar at the bottom
Tap the Repeat icon
Tap once to repeat the current playlist or album
Tap again until the Repeat icon shows a 1 to loop the current song
On desktop, click the Repeat icon in the playback controls
Click again until it shows Repeat One to loop the current song
To stop looping, tap or click the Repeat icon until it turns off