Determine the chords you want to use in the piece. You don't need to use traditional chords that are built on thirds, such as D, F and A. Instead, experiment on the piano with clusters of notes that sound pleasing to you. Choose four pitches for your first cluster of notes.
Create a rhythm that you will use throughout the entire composition. The rhythm should be repetitive and have a nice pulse to it. Avoid choosing anything that is too complex.
Slowly change the harmonies by moving one voice at a time by step. You can move in any direction you want, but avoid crossing any other voice, and use skips and leaps only when you get stuck. For instance, if you have the notes, C, E, F and A, then move the top note, A, up to B-flat. Then a few measures later move the F to F-sharp. Gradually move each chord voice over the course of the entire piece.