Most after market piston rings are 'file to fit'. To say that, they are manufactured .015 to .020 oversize to fit differences in the way each cylinder is bored.
I don't have photos handy, but you take each cylinder individually and each piston (I use that cylinders piston) and a set of the three rings that you intend on using in that cylinder. I then push the top two, one at a time, about an inch into the piston bore, using the top of the inverted piston itself and measure the end clearance with a feeler gauge. This should end up being be.005/.007 per inch of piston diameter(4" piston would be .020 gap) for the top two rings AFTER YOU FILE TO SIZE. This may entail several 'trips' into that cylinder and several passes with a file or special filer tool (Summit and others will have that tool). The bottom and usually 3 piece ring doesn't get any fitting (normally) and is installed as supplied.
There are many write ups on the internet, but if you have conventional cast/moly rings, will all use this kind of assembly. There are other kinds that are installed differently, but are specials with those 'special' high buck prices.