First, we need a source image. Use Google Images to search for a car of your choice, and when you've found one, copy it and paste it into a new document in Photoshop. For this, I'm going to be using this car:
First of all, you're going to give the car a paint job. I'm going to give this car a dark red colour. To do this, make a new layer and set the Opacity to 50%. Select the Brush Tool, pick out the colour you want to use for the car's base, and paint around the whole of the car. Make sure you only paint the bits that need to be painted, and not the rims, windscreen, etc.
You could use the Polygonal Lasso Tool to select around parts of the car that need filling in with the base colour, it can be easier that way and much faster. If you do go over parts of the car which are not meant to be coloured, use the Eraser Tool to get rid of them. When you think you've finished that part, you should have something like this:
When you're happy with the paint job on it, click on the top layer and press Control + E to merge them together, merging all layers into a single one, obviously. You don't have to, but it's easier to work that way.
OK, you're now going to tint the windscreen and windows. You don't HAVE to do this part, but it makes a lot of difference if the windows are looking somewhat "jazzy" and better looking than the original windows. It's up to you what colour you choose to paint the windows, but I'm going to use a simple black colour to just darken them up a little.
Make a new layer ontop of the original one, and set the Opacity to 40%. Again, use the Polygonal Lasso Tool to select areas if it helps. Paint around the windows and the windscreen (and the roof window, if the car/vehicle has one, and in this case, the car I'm working with does.) Once you're happy with what you've turned the windows and windscreen into, it should look something along the lines of this:
OK, Google Images is going to be your main help here. This part is quite tricky, probably the hardest part of the whole tutorial. Now, use Google Images to search for a car/vehicle that is the same shape and view of the car you're working with. It has to be at the same angle (or near to the angle), otherwise it won't work.
I'm going to use this car, cut around the rims, and put them in place on the car I'm working with:
(As you can see, the car is at the same angle and looks the same shape to the car I'm working with.)
Do whatever you have to get the rims at the same shape and angle. Free transform the picture, rotate it, and cut out the rims then place them on the car/vehicle you're working with. When you're finished with that it should look similar to this:
Now I know it doesn't look the best but I think they're better than the original rims. And plus, it's a real hard job to do, especially finding a car that is at the same angle and shape as the car/vehicle you're working with, let alone editing it and transforming it to fit onto the car/vehicle itself.
Alternatively, you could forget about adding new rims... What about painting the rims that are already on the car? Let's do it. Again, merge the payers (Control + E) so that you have less layers and complications. After that, make a new layer, set the Opacity to 40% and paint over the rims that are in sight on the car.
Again, it's up to you for what colour you choose. But I'm going to use a dark black. When you're satisfied with what you done, check to see if it looks something like this:
If it does, then you haven't got long left.
OK, we're now going to edit the lights of the car. You can either do what you did with the rims (finding a car at the same angle and shape, cutting the lights off and editing them to fit on the car/vehicle you're working on), or paint over them in any colour of your choice.
You get the jist of things when it comes to adding new things from another car that you found, so there's no point in showing you that again.
Once again, press Control + E so that all the layers are merged into one single layer. Now, make another layer, set the Opacity to 40% and paint over the lights/headlights with the colour of your choice. I'm going to use a grey-ish sort of colour.
Once you're happy with it, you should have something that looks like this:
Congratulations! If you've got this far then well done!
I know some of you may be thinking, "But the pimped result of the car doesn't look any better than the original version." Well, I'd like to think that my finished product of the car looks a bit better than the original source image. But I made this mainly to show you how it works, and for you to get the idea of how it works.
I think I've provided all you need to know in the tutorial, it's your turn to take it from here.
Hope this helped you. And could I ask everyone who tries this to post their finished result if they can. Before and after. Thanks.