A lot of the time I'll put something on paper before I try it on a flag, but that's mostly for the overall design goal. Once I start painting, I might mess up somewhere and have to adapt. Make something out of a mistake.
Take this for example. While it's certainly not great, it works. I just started drawing lines and made a pattern that would have plenty of elven "Swooshes" on it. I accidentally had a glob of paint in the middle, so I incorporated it into the design, filling in a few gaps.
The main point is just to have something on it. Looks SO MUCH better that way.
My armies: Empire, Dwarfs, High Elves, Wood Elves, Dark Elves, Lizardmen, Skaven, Tomb Kings, Ogre Kingdoms, Orcs and Goblins, and Skitarii (40K).