My family is into cycling, A LOT! We live next to the Minuteman Bikepath, the busiest bike path in the USA. We use part of our basement as our cyclery (or bike boudoir). And it looked like a dungeon down there!
So we decided to spruce up the basement with some floor paint, and a mural.
You need some stuff:
1) Some art - your own original or a poster perhaps
2) An overhead transparency projector - or a modern PC projector
3) Some paint (latex house paint is fine), you can scrounge from your paint stash or buy small inexpensive samples at the hardware store.
Step 1: Choose the art you want to use.
Step 2: Create a transparency of your art
In photoshop, we simplified the picture even further and approximated the aspect ratio of the wall, which is the ratio of the width against the height of the wall. That way, the wall would be completely covered by the mural image. Don't worry about perfection, you can easily extend lines once you start tracing and painting.
By the way, printing fine detail on a transparency can be tricky if you don't have specialized equipment. The main problem is that the ink doesn't want to dry. We didn't have a good printer and it's ok as long as you don't print anything complicated! This is another reason to keep the design simple.