Sometimes my ideas have to cook a while, upstairs. I'll think on an idea for a good while sometimes before I actually start it. At times the ideas, the process of HOW to make something work will come together quickly, other times I sit ideas on the shelf and as new ideas come I put them together in my head.
I used to hate it in college when we had to sketch out our ideas first, I just don't work like that.
I usually come up with more ideas while I'm working, like when I made Beanie the CANimal elephant. At first she was just going to have a decorative tapestry on her back, but THEN I thought, "No! this needs to be functional!" So I turned it into a wallet.