Here are some links to resources I have found useful when doing mathematical origami
https://wild.maths.org/tags/modular-making – Some great videos from the Millennium Mathematics Project in Cambridge showing how to make some simple modular origami models
https://think-maths.co.uk/resources/make-dodecahedron-a4-paper/ – lovely set of instructions for making a dodecahedron from A4 paper
https://mathigon.org/origami – the Mathigon website has lots of gorgeous pictures of origami models together with instructions for how to make them, including the Five Intersecting Tetrahedra I blogged about here.
http://www.origami-instructions.com/modular-sonobe-unit.html – instructions for my favourite modular origami unit, the Sonobe module
https://plus.maths.org/content/trisecting-angle-origami – an article explaining how origami can be used to trisect an angle, which is impossible using standard straight edge and compass techniques