Making a loop after you have made a cord:
Making good and strong plant fiber bowstrings that are simply two plys takes skill. Too much bark left on or thin sections will make a string weak. An option for the beginner is to take cord you have already made and double it over and reverse twist that into a bow string.
Plant strings whether they are single ply or double ply, in my estimation are thicker than normal modern arrow nocks. I think that helps explain Native American y nocks or other nock variations that allow for a thicker string.
This method will more than double the original cords test poundage. Even though you are doubling the cord over, Reverse twisting helps more than double your strength because you are not only over coming the tensile strength of the fibers you also have to over come the friction of reverse twisting to break the string. However, because you are reverse twisting again you will loose roughly a third of the length in your final product. What I mean is a 20 feet long cord doubled over and reverse twisted will not yield a 10 feet long cord end product. It will be more like 7.5 feet. Always consider the length cord you need, then double that number and multiply by 1.33. This will give you plenty.
If you come up short like I did on the last picture. Simply untie the knot for your original cords and splice in some new segments until they are long enough. You may have to unravel a few inches do do this.