I can’t do a step by step right now, but I can tell you how I did it overall.
I first took the screen out of its casing and measured the length and width of the screen, I then got some balsa wood ($1) from the hobby shop down the street and made a draft of what I wanted it to look like in the car. That type of wood is thin, light and very flexible. The best part about it is that it is very good to work with you can use wood glue and you can cut it with scissors so you can build it in your car with out any big tools.
Once I made the form the way I liked it I went to the local car store and got some fiber glass ($6) and some fiber glass mat ($4) and put 2 layers on top of the form that I made.
I then sanded it down, made it look nice and then I covered it with Bondo ($5 Wal-Mart) Bond gives it a really nice smooth surface to paint, you want to get all of the pin holes filled, I didn't get all of mine out before I painted it and I really regret it.
I then went to the local car store and got some spray paint ($4), but I got the fake leather paint to give it the same look as what is in the car. It came out really nice for the first time; I plan on doing another one in the spring when the weather gets a bit warmer.
A really good site for this is
http://www.mp3car.com, everything you ever needed to know is right there, I have been reading this forum for over 5 years now, and it is great and the people are really nice and helpful. Just make sure you do a search before you ask a question.
The total cost for my project was a little less then $700 and is worth every cent, you would pay that much for just a decent GPS device, this way you get a lot more for your money.
If you need any help send me a message and I will be more then willing to help you.