Had to wait 25 years to be able to do this ... But it's my house now, and my swagg goes on the walls ;-)
Just had to paint my son's room, since it's the old room of my daughter, and it had pink clouds in it, and for my son ... just ... no way ... had to be done ;-)
Step 1: Prepare the Walls, the Color, the Style ...
First of all decide what colors you like. I like to choose my colors based on the furniture, carpets and ornaments of the room. It's easier to choose the colors than find furniture which matches the room colors. Same with the style: modern furniture -> modern paintings etc. In my case I went with: "modern, sport and and hopefully my son still wears it proud when he gets older"
For planning I use Inkscape. It's the best free vectorial drawing software and served me well so far. Choosing form and color is very easy if you see it on screen. More about it in the next step on the actual paintings.
For painting the background there is no complicated technique. The secret is masking tape, lots of it. After marking the lines on the wall with a pen just cover the margins of an area of the first color you want to paint. Wait for the paint to dry and do the next color. Finally it will look like the pictures.
If you are finished with the background, you may want to add some drawings. They give the room a more personal touch, based on the personality of the person.
As I philosophised about the personality of the boy he is now, and what I want him to become, and how everything should be the base of the style I want to put on the wall ... my son said he want's a car ... well that works too ...
I showed him a lot of cars, and he chose the one in the first picture of this step. I dragged it in Inkscape as a background and drawed the one besides it (find svg file attatched).
For the next part I used a projector and my camera to project the image from Inkscape on the wall (pic 2). Then I drawed the outline of the care on the wall (pic 3) which ended up like pic 4.
Using a set of brushes of different sizes I painted the two colors of the car on the wall.
Same I have done some time ago for my doughter. The same technique was used.
First I painted the background, all the rectangles of different colors.
I used Inkscape for drawing the forms (attached), projected them on the wall, and painting a lot.
Over time I made more of them.
It doesn't have to be a drawing of your own. You can already find tons of vectorial drawings, clipart, drawings in general in the internet, which you can directly use.
Instead of the projector, you can also use an overhead projector, or a lamp and a printed transparent film.