I found the tutorial for this at
Mine doesn't look the greatest but it will do for my room.
You can go the website for the tutorial or read mine here.
The elastic was dead in my fitted sheet so I cut it out and tore along the grain line to get straight sides
I made cornstarch paste by mixing 1/2 cup of cornstarch with 8 cups of water. I whisked this together on the stove on medium heat until it became clear and had no lumps and had thickened.
After it had cooled I got my sister to help me hold up the edges of the sheet so they were aligned with the edges of my wall and I painted on the mixture and smoothed all the bumps out.