Start by getting a medium sized mirror. I bought mine at target ages ago during Halloween. It was about 20$
Find the image you would like to put on the mirror or glass surface. Print it out on paper. DO NOT USE GLOSSY PAPER!!
Take your exact-o knife and cut out the image.
Clean your mirror or glass surface with a glass cleaner. Make sure it dries before placing the image.
Take your Elmer's glue and apply it to the back. Use this sparingly. You only need a thin coat to cover the back of the entire image. I find it best to use a small paint brush.
Next, apply your image to the mirror or glass surface. Make sure you place it right the first time. Pat down the image until it is flat. Clean any excess glue with a warm, damp cloth.
Let the mirror/ glass dry over night. To clean the mirror in the future use Windex on a rag or get glass cleaning wipes. You don't want the image to get wet or it will run.
*Please excuse all of the crap hanging from my mirror!*
If you decide later that you want to remove the image you can take it off with a straight razor and goo gone.