Its REALLY easy!!!
I did it in my bedroom when I was about 14!
You can do multi colours too which might work well with a mini
Go to a craft shop (hobby craft is good)
The best stuff to use is plastic, like the kind that folder dividers are made of, you know flexible but firm?
Then get a craft knife and use that to cut out for example the main body of the car which will be one colour.
Then another for the wheels, and another for the glass?
You can do it all on one, but doing 3 means you can overlap/get colours right next to each other - whereas doing it all on one peice of plastic means you have to have a peice of plastic between - which is trickier to paint!
So you have your templates - then get tester pots for the colours, and you need stiff brushes - stippler or something its called
Draw faint pencil marks to mark out where to top of each car goes etc
Then place the template in the right place and can either hold it or stick it with masking tape.
get a small amount of paint on the brush and dab it gently, always moving inwards from the plastic to the middle
Move along the wall and do the next, then when its dry do the next colour
Its really quite quick and looks nice- and you'll be pleased you did it!
I did a 3 colour one round the top of my room and it was fab!