How to use the cock & ball ring: 

Put ring on flaccid or semi-erect penis. Use water-based lubricant if needed. Slide cock ring down toward the bottom of the shaft, then carefully slide testicles through ball ring, one testicle at a time. 

Small ring diameters: 0.9" & 1.1"

Medium ring diameters: 1.1" & 1.3"

Large ring diameters: 1.3" & 1.5"