These days, the typical Hollywood way to handle a complex car like the Batmobile is to model it and simulate it with a computer. Even Yoda and Gollum are modeled and simulated on a computer -- a car is a piece of cake compared to Yoda.
This isn't how it works if you are Nathan Crowley. With "Batman Begins," Nathan tended to be a staunch realist who wanted an actual, physical manifestation of the car in every frame of the film. Therefore, Nathan started the process of creating the Batmobile by model bashing.
Model bashing is a time-honored technique. You go down to a toy store, hobby shop, R/C specialty shop and the hardware store to buy parts -- lots of parts of every size and shape imaginable. You buy lots of plastic models, toys, R/C car kits, metal tubing, etc. You then cut and shape all of those parts to get the desired look for the car. For example, Nathan found that the nose cone from a plastic P-38 model kit made a perfect shape for the jet engine on the back of the Batmobile. So he cut off the nose cone, hollowed it out, added in other parts to make it look like a jet and glued it onto his model.
Nathan built six models like this, all 1:12 scale, before he got the look and the shapes that he wanted. This process took about four months.
Once he had the scale model, he started on a full-size replica.