In the 50s there was a kids toy that would show a photo on the wall, it was perfect for this kind of thing. It was just a black box open on the bottom with a light, a mirror and a lens and you just set it over the photo. I haven't seen one for years.
If you have a full size pattern there are other tricks.
1 pin the pattern to the wall and place graphite paper behind it and sharpen a stick to trace the picture leaving fine lines on the wall
2 pin the pattern to the wall and with a pin poke little holes thru the pattern leaving little holes in the wall to follow with paint.