To make a loop on an iPhone, you can use the built-in Loop feature in the GarageBand app. Open the app, create a new project and import the audio file you want to loop. Once the file is imported, tap on the track and select the "Settings" button. From there, turn on the "Cycle" button and adjust the loop points to your desired length. You can then play the loop by tapping the play button, and it will continue to repeat until you stop it.