You can google Ball Stretching and find sites that have safety notes.
Safety first!
The testicles are a delicate organ, and damage can easily be caused. If any abmormal signs show, then play should be stopped immediately, and if any pain continues significantly after stopping, then a doctor should be consulted. Abnormal signs include swelling, discolouration (especially purple), pain, cold or numbness.
Any bindings or devices used to hold the testicles must not be so tight as to cut circulation, and must be removable immediately on any problem.
Effects on fertility are unresearched, but believed to be on the destructive side. If you plan to father children, you may not want to partake - especially in extreme measures.
http://everything2.com/e2node/ball%2520stretching
http://wiki.bmezine.com/index.php/Scrotal_Stretching
I personally started with using 0-rings found at the hardware store or on adult sex toy sites. Multiple rings to require practice but can be stretched to remove.
I also like the wide rubberbands that you get when you buy broccoli or on lobster claws. A nice way to stretch out the sac and get a skin tight on the balls.
Found a napkin ring that worked well but the metal was a little thin and chaffed a bit.
The solid rings need to be worked up to.
Split Collar rings have the ability to be taken apart with hex wrenches. A good safe way to go.
I have found some interesting instructions on trussing up the cock. Played a little that way.