This is a graphical representation of how I do it...
first stage- find as much reference pics that will be used as possible, preferably white, nice angles, easy to work with
second stage - unwrap the model, render the UV map at 512x512, throw into Photoshop
third stage - start applying pieces of the picture onto the UV map, constantly checking the model if everything aligns, I also draw up some stuff like the bonnet and roof lines, some textures like the front bumper are just tiled patterns etc.
final stage - checking if everything aligns, making window textures, applying other textures, cleaning up, bugfixes, paintjobs
[/list]