Yes, there is not much extension documentation at this point as there is so much development going on in that area of the system. We would value volunteers to help with writing them!
If you want it to be included in the checkout and order pages you need to chat with @sampoyigi regarding the cart extension (https://github.com/tastyigniter/ti-ext-cart) and whether he sees it as something he would want added.
As for a standalone extension, you can follow the approaches used in the other extensions - just look around the codebases and learn from them. You would need a routes.php file I would imagine to send customers to an url of your choosing. I’m happy to answer specific questions if I can.
