4♠ Start with a loop

4♠ Four of Spades in The Rock Songwriter’s Deck: 52 Ways to Write a Song
The Four of Spades gives us a time-honored way to write a song. Start with a loop. A loop is a segment of audio that repeats over and over. You build your song over the repeats of the loop. The repetition gives you an automatic catchy element, while it also enforces a constraint on where your song can go.

In olden times, before music was recorded on computers, people still worked with loops and build songs from them. They cut out a section of magnetic recording tape, spliced the two ends together to make a physical loop of tape, and arranged the tape recorder so that it would keep playing that section of tape over and over.

Today, it's almost too easy: you can grab any audio clip and tell your recording program to loop it. (Copyright warning: It's a good idea to use audio material that you're legally entitled to use. If you sample another artist's song, you might create legal troubles that cost more than your new song is worth.)

If looping digital audio doesn't suit your style, you can also do loops the acoustic way: play a repeating riff or chord phrase over and over without variation, and use that repeating material as a framework on which to build a new song.