Does anyone know how to make one of those doormats make of old flipflops? I can't seem to find instructions

I've never seen one.

If it is what I think you are talking about, they seem to just be small uniform pieces of flip flop material (or any rubber product, I have seen the same stuff made of old tires) with holes in it. Then uses some sort of cording to weave and tie it all together... I think it would be incredibly labour intensive to do it by hand, and you would probably end up with stitches from a utility knife wound.

They're not made of old flipflops, they are made of scrap material from the process of making flipflops.

They must have a press that cuts uniform pieces and another process that threads them together.