1.21.5 data driven textures

I know it became possible to implement data driven textures into resource packs, but when I tried finding out how to do it, I either got outdated results or just mentions of it being possible. How can I add a data-driven texture?
Just for an example, let's say I wanted a data driven texture for an "iron headband" for the iron helmet to turn into when renamed. I make the equipment and item textures... and then what?