You can invent a story or describe an event. Or you might prefer to begin with your immediate surroundings and see where your imagination takes you. You could just write a series of unrelated sentences. The important thing is to use only one-syllable words. No exceptions.
When you do this exercise, especially if you do it more than once, you will notice different possibilities for rhythm in language.
You will notice entirely new rhythm patterns if you write a page using only two-syllable words. Or three-syllable words. What about four syllables? When does it stop being a writing exercise and turn into a language puzzle?