Rolleiflexes often came with a leather "EverReady" case - often dubbed "NeverReady" because they are hard to put on and take off to do something as simple as put in a new roll of film. Mine is old and cracked and useless, so I needed a new strap and this is the one I made.
Get a strip of latigo leather - this is the type of leather that belts and straps are typically made of and is stronger than some of the other types.
With a skivver (a sort of leather shaver) or a knife or a plane (what I used), thin the leather down at the ends until it fits through the guides on the side of the camera.
Punch a hole the size of the post and then make a slit above the hole to make room to fit it over the head of the post.
Then slip the leather through the guide and work the post through the hole and slit you made.
I made two short pieces which fit on each side and joined them with a buckle to make a short hand strap. I also made a longer piece which can buckle onto the ends of the short pieces to extend it and convert to a neck strap.