The process starts with photos of the car. If you are lucky enough to be working on one of the all-white model Scalextric and some others release it is a good idea to take these photos before you disassemble and strip the car. If you are not so lucky it might be best to disassemble and strip the car first so that you have a blank canvas on which to work.

They don't have to be brilliant photos, phone snaps will do, but make sure your photos are as square on as possible. These will form the canvas upon which you build your design. You will want to take a photo of each side that you plan to put decals on.
With photos in hand open your favourite graphics program, create a new blank project and import the car photos.
It's possibly worth noting here that I use Illustrator for my design work, but the same steps will work in vector or raster based programs that allow use of layers (or pages in CorelDraw). I would recommend not using MSWord, Publisher or Powerpoint, for the process, although it would probably work. There are several free opensource graphics programs out there which are very good.
Run a measuring device over your model and scale the photos up to full size of the car. By doing this now, when you get the logos right on the layout you will know that they are the right size to print and put on the real car.
This is how I lay mine out.

Sort of like the car is an unfolded box. It works for me, but something else might be better for you.
Once you've got this far it's a good idea to lock and protect the layer that the photos are on. It will save you from accidentally moving or resizing the photos. It's also a really good idea to SAVE THE FILE NOW.