Old versus new
Always get a new wrist pin bearing. They are cheap and you don't want to chance one going bad to save a few bucks, you'll be in there anyways so just slide the old one out and slide the new one in.
Install one circlip, then the wrist pin into the piston and through the bearing, then the other circlip. I just use a pair of needle nose pliers to get the circlips in.
Make sure the arrow on the piston goes towards the front of the bike.
Here are the rings installed. They have to be lined up just right in order for the cylinder to slide back over it. I usually have someone lower the cylinder on while I hold first the top ring then the bottom ring in position. There is a small pin in each groove that lines up with a notch in each ring. It really won't go together any other way. It's also critical at this point that as the cylinder is lowered down that the notch on the power valve line up with the pin on the matching linkage from the trasmission. Power valves differ somewhat between models so this step may not need to be done on different bikes. Make sure to tighten the bolts in a criss cross pattern slowly to prevent warping. After everything is assembled the operation of the power valve can be checked by taking of the cap on the side of the cylinder and watching the notch rotate while the engine rpm's increase and decrease.
Always clean out your power valve as part of the top end. If you don't, it will end up sticking and causing poor power characteristics. Mine was starting to stick before I rebuilt it last June. It would usually rev up fine, but would sometimes take a second to get my low end power back. It felt like I had about half power until I blipped the throttle to get the valve to close. Also check your reeds each time to make sure they seal correctly and are not cracked.
That's basically it! Although this was a 2001 CR250, the same basic procedures apply to most 2 strokes. If you still have questions, Here is a Short Video that shows a detail top end on a yamaha 2-stroke bike.
