Theme is probably the most important part in composing aleatoric music. Without a theme, the music you create will have no central motif. So in order to compose a perfect piece of aleatoric music, create a theme before doing anything else.
The theme you created will become the core of the aleatoric music that you compose. Expand on the theme by compose a wide variety of variations in a different key. You will need this in order to create the overall composition of the music.
When you are done creating the composition, you can move on and add aleatoric elements to your composition. Compose the piece of music in such a way that it contains plenty of options (chances) for the performers through its entire length. You can include choices such as changes in key, tempo or time signature. Make sure that all the options you add are valid musically and are listed on the composition sheet in a series of option such as A, B, C and D so that the performer can choose from the options.
You should add a number of occasions in the composition where the singers can repeat words randomly. This technique to compose aleatoric music was initially used in ‘Cloudburst’, a famous piece of aleatoric composition.
Create randomised sequences in the aleatoric music you are composing using sequencer's random number generation software. The software will capable of creating an infinite number of variations in the song.