Flexagons are hexagons made of card or paper that you can turn inside out, so instead of the 2 sides you would normally get with a cut out hexagon, you have 4 sides!
You start with a strip of paper and mark out or fold a triangle with equal length sides. If you have a protractor handy you can mark out 60 degree angles all along the strip.
From the first triangle zig zag the paper back and forth.
You only need 9 triangle so cut off the others.
Now you need to fold it into the hexagon shape. The creases will help you here as you never need to go against an existing crease.
To help you see where the folds should go try to imagine a rhombus or kite shape on each section.
Tape the loose section on the edge
There are now 4 sides to a 2 dimensional shape. If you draw on the outside edge it will appear on the inside edge of the next rotation.