Usually, I just add parts as I think of them. I, also, tend to write in my own format (see below).
(Steve wakes up. After taking a shower and shaving, he goes into the kitchen and gets a glass of tea, then returns to his bedroom and boots up his computer. A fuzzy, little grue appears out of thin air, next to him.)
Grue: What's up?
Steve: I'm bored.
Grue: It's too early to be bored.
Steve (yawning): I know.
(Lightning strikes and knocks out the power. The lights go out. A mysterious, disembodied voice is heard.)
Voice: It's too dark. You are likely to be eaten by a grue.
Steve: Ducky.
Grue: Here, hold this.
Steve: What is it?
Grue: Flavoring.