First make sure the pistons are facing the right direction.
Make sure to keep the cam chain tight by holding up the cam chain with a bungie chord or have someone RELIABLE to hold up the cam chain while you do this.
You can simply lower the cylinders over the pistons and starting with the middle two or outside two pistons, put a block of wood under the cylinders to keep the cylinders from falling on your hands.
The using your fingers squeeze the piston ring from all around the outsides and push the piston up one ring at a time into the bore. use engine oil on the cylinder bores. Do NOT use WD40!
OR...
You can use a large radiator clamp around the piston ring to squeeze the piston ring and push the piston up into the cylinders.
The first method is easier because the radiator clamp is a pain in the A$$.
Make sure the piston rings are spaced with the gaps 120º apart from each other.
Also make sure you have the rings facing up on the piston. Marked with a T on the top.
When you open the package of rings they are usually stacked in the right order unless you start
playing around with them.
Make sure to watch this video.
http://www.youtube.com/watch?v=9ll19_DSkTM
On a Roadstar Adventure.