Not sure if its all there, if so I think you put the gun and leather part over your shoulder and on the carrying side, and the strap goes around your back to clip to it.
Goes over your head and shoulder so youll have the leather across your chest. Then the white strap goes around your belt and anchors it down. I never seen that particular one but from looking at it thats how I would assume it goes.
I have one quite similar, it looks to me like your missing the piece that goes around the belt on the non holster side to keep it from riding up and falling off the shoulder.