https://alexdrevessa.com/2019/10/31/tricky-hexaflexagons/
I love the Vi Hart videos on these! I broke down the steps and slowed down the process to make it easy for the first-timer to follow. I also pointed out common pitfalls, based on my experiences in teaching a few dozen kids how to make these. Vi Hart's excellent video on these is here: https://vimeo.com/147799851 (For the curious, no, I didn't cut myself-- I gave blood that afternoon, and the purple Band Aid is from the finger-stick.)
The next step is the hexa-hexaflexagon: https://www.youtube.com/watch?v=JDO-JIjoIlk
If you're a teacher, here are three lesson plans on these: https://alexdrevessa.com/hexaflexagons-three-lesson-plans/
