On the picture below is the construction that I use. It is constructed from three chains. To the left is the neck chain. On the right side is the chest chain, which actually consists of two separate chains. Everything is held together by the key ring in the middle. The bigger rings lying separate belong to the ellbow chains. The purpose of the small key rings (connected to the big rings) is to make it easier to connect the chest chain to the ellbow chains. The length of the neck chain should be such that its end is at the point where the ribs end (or a few centimeters lower). Making it shorter will cause unnecessary pressure on the neck, making it longer will cause the chest chain to press on the stomach. The length of the chest chain should be such that your elbows are fixed at the position where they naturally are, when your hands are tied behind your back. Forcing them at another position is unpleasant and causes circulation problems. If the neck chain causes too much pressure, you should probably make the chest chain longer.