The first thing that you need to do is to decide just where it is that you are going to be making this mural. Now your room is going to have a few walls in it, so based on how the room is set up, you need to think this through.
You can decide if you want to draw it behind your bed, on an empty wall, or on one of the walls on the side.
After you have gone on to figure out where to make the mural, the next step is to figure out how you are going to be making it and just what it is that you are going to be making. This is the most interesting part, since at this point in the process, you go on and finally decide just what image it is that you are going to be drawing.
After you have figured out just what it is that you want to draw, you can go on and start to clean the wall. Clean off all the dirt and particles that could interfere with your art, since you want a good clean and smooth surface to draw on.
With the wall now being cleaned, go ahead and draw on it. Use a light pencil at first, so that you can erase it and start over, should that be required of you at some point.
After you have laid out the basic art design, simply go on and draw over it with a darker pencil or a pen and you are now good to go.