Pick a mood for a starting point, or set the key, scale, tempo and length yourself. Press Play: the music is made as you listen.
Each layer has its own seed. Reroll (the dice) gives that layer a new idea and leaves the others alone. Lock keeps a layer's idea when you reroll everything else or pick a new mood; it still follows the key, scale and tempo. A good routine: lock the Harmony, reroll the Melody until something grabs you, then shape the Arpeggio's rate and activity.
The Arc decides how density and register change over the length of the piece: a Swell rises and falls, a Build only rises, a Fade thins out, Tides come and go.