Start sticking the lance-shaped or basic leaf shaped paper pieces on the background.
You can outline the design of your art before starting, but I think it's fun not to. I started randomly and kept going. At the end it turned out beautiful itself!
After you're done creating the main design then you can stick few more small paper pieces to fill empty corners (if you want). I also glued small paper pieces over the big ones, to make the wall art look more colorful.