I bought a cheap pair of white flats to decorate. At first, I was worried that the vinyl-like material wouldn't work, but it worked out just fine.

Cut out any images you want to use. I used one comic book (Harley Quinn #4) that had a lot of pictures that I wanted. Cutting probably takes the most time, so just be patient. Try to cut some pictures so that they have a background, and some with just the character you want, so you can overlap them nicely. Also make sure they generally fit the shoe (but you can snip more off as you glue them on)

Now, apply some mod podge on the area you are to place the paper. Put it on, smooth it out, and brush more mod podge over the edges. Repeat for both shoes.

Lastly, put on a thick coat and let it dry until clear. I put on two more after this to be safe, and let it dry for 72 hours after the last coat. Outdoor mod podge should make it waterproof, but don't go jumping in any puddles or something.

Enjoy your shoes!