An excerpt from my BBK how to... should cover most questions about rings...
Start by determining where your piston rings should be installed on the piston. You should have a total of five piston rings. Two thick metal rings, two very thin rings, and one wavy piston ring. The two thick rings are called compression rings, the other three are often refferred to as one oil control ring because they all fit into one ring land (thin rings on the outside edges, wavy ring in the center). As their names imply, the top two rings are primarily used to control compression and seal the clylinder and the lower ring(s) is used to control oiling.
Take a close look at the compression rings. They are usually meant to be installed in a certain order. Sometimes they are colored to let you know which is the top ring and middle ring and sometimes they will be labeled with a "1" and a "2" or other markings. A common coloring is one black and one silver compression ring. In this case, the silver ring should be the top ring and the black ring is the middle ring.
While you are checking out the compression rings, take note of any markings. Only one side of the ring should have markings on it. This is the side of the ring that will face up when installed on the piston.
Now that you know where all the rings go, it's time to set the ring end gap on the compression rings. Ring end gap is the distance between the ends of a piston ring when it is installed in the cylinder. An end gap that is too small can cause cylinder scuffing and too large of a gap will cause poor sealing and excessive blowby.
You'll need to determine the distance to set the end gap. A good rule of thumb is to set the end gap for the top compression ring to .004" per inch of cylinder bore. Set the second end gap to .005" per inch of cylinder bore.
Top Compression Ring End Gap = Bore Diameter" x .004"
Second Compression Ring End Gap = Bore Diamter" x .005"
Since these measurements are done in inches, you'll need to convert your bore diameter from mm to in.
Bore Diameter In Milimeters / 25.4 = Bore Diameter In Inches
For example, my 72cc big bore kit uses a 47mm bore.
47mm / 25.4 = 1.85"
1.85" x .004" = .007"
1.85" x .005" = .009"
I'll need to set my top compression ring's end gap to .007" and the second compression ring's end gap to .009".
To set end gap, begin by placing the piston ring in the cylinder. Make sure the marks are facing up just as it would be installed on the piston.
Use the piston to make sure the ring installed straight so you can get an accurate measurement.
Now use feeler gauges to measure the distance between the ring's ends.
If the distance is too small you can file the ends of the ring and recheck. Be careful not to damage the ring. Apply light pressure and try to maintain the original angle of the ring's ends. If the end gap is too large your only option is to get new rings or if they are close to the gap needed, just use them.
Once your rings are gapped properly, it's time to install them on the piston. I suggest using a piston ring expander tool. You can pick one up pretty cheap from most auto parts stores or possibly rent one long enough to do the job.
Start by putting the top compression ring in the expander tool and opening it just wide enough to fit over the piston. Be very careful not to spread the ring too far and break it.
Move the ring into position in the top ring land and slowly release the pressure from the piston ring expander. Make sure the ring is in the ring land all the way around the piston. Some people like to use assembly lube or two stroke oil to coat the piston and/or rings for this process. I do it dry.
Repeat this procedure, placing the 2nd compression ring into the middle ring land. When installing the oil control rings, make sure the two thin rings are on each side of the wavy ring. It is often easiest to install the wavy ring first, then the thin rings on the edges.
Most of the risk of breaking the rings on these things is installing the rings on the piston. If you take your time it's really no trouble sliding the bore over the piston, but if you can find a compressor that small it wouldn't be a bad thing to have. You can never have too many tools.