They are very simple to make and there is no need to think you must copy this one exactly.
Simply start by carving the concavity for the bow limb. Once that is shaped then make a shallow groove for the rope to run in that coils around the 'head'and onto the shaft.
when using it don't exert too much pressure as a lot of pressure also makes the parts twist. You can clamp both tips to try and stop this but notice the man in the video doesn't actually exert much force. This is the trick with this tool.