Make sure you stuff a clean rag around the rod in the crank case after you pull the jug off. You don't want anything falling down into the crank case. After installing the new ring on the piston, align it with the pin, put some oil around it, squeeze it tightly onto the piston, and carefully slide the jug back down onto the piston and ring. Oh, take the rag out. If you are finicky (and there is any slop in the cylinder bolts), there is a cylinder alignment procedure you can do before tightening the cylinder retaining bolts.