I used the program MeshMixer to separate the shells and isolate just the bookmark template from the original Unicorn Bookmark. I then used the program eXtrudy to create an object from an image then combined the object using MeshMixer create the final stl file.

Guidelines for a good bookmark:

1. Make the top object no thicker then 3 mm and about 70 mm x 70 mm at the tallest or widest. This is easy to do in MeshMixer using Edit > Transform > Size X, Size Y, or Size Z.
2. You may need to extend the top of the bookmark template a little in MeshMixer to make everything line up so it looks nice. (Select top face > Edit > Extrude)