We play every other week. So I tend to dream stuff up for the first 10 days and then put off writing stuff up until 2 days before.
Then tweak it before I print it out. The Monster Builder really makes it much easier to build what I want. This week, I wanted some Fomorians, but they were too high a level. 15 second later, they were the correct level. It makes it so much easier to fit what I want in.