My kids enjoy making flexagons from scratch, and there are several sites with pre-made designs, but I could not find any generator for making your own printed designs for the simplest hexaflexagon.
So I made one, written in PHP using GD and FPDF, and you can use it right here!
All you need is 3 images and a printer. The images will be cropped and scaled from the centre to make them all the same sized squares, before being divided into 6 triangle segments forming hexagons. If you want maximum control you should load in 3 square images. The generator accepts png and jpg images only and the output is a pdf. If you select the correct paper size for your printer you should have no issues with stretched images. There is also an option to see what the cropped scaled images will look like before you generate the pdf.
Here is an example, made in just a few minutes! 3 free space images… 1, 2, 3, and the output!
I first read about hexaflexagons years ago in one of the many Martin Gardner books. This one is available on amazon. Hexaflexagons is the first chapter, and can be partially read using the Look Inside feature, including instructions on how to fold.