Below are three clips from youtube. Download one or all and extract the sound this can then be used that for your 20 second cut up.
You can mix them or just use one. Utilise the method for the cut up techniques as outlined by Burroughs, you can cut out phrases, layer sounds, repeat, slow, speed up, reverse.
You have heard some examples of sound experimentation by Steve Reich in “Its gonna Rain”
Listen to some of these examples and experiment in creating your own cut ups from the sound files below.
A simple tool for extracting sound from any source is a piece of software called Piezo from Rouge Amoeba. Use the free version and you can record up to 10mins without restriction.