Well, mind you, the kitbashed models come first, and then I draw the blueprints after the models.
The models come about in many different ways. Sometimes I just happen to notice that parts will look interesting shuffled around this way or that. Sometimes I start out with an idea and struggle to make the ship fit the idea. Sometimes I'll look at something that's not even close and consider it a challenge to make a starship out of it (See the Val'Walo!). It's very rarely planned out in advance, mostly just thrown together as I go. You're right, it's not easy to answer.