Here are the overall steps I did for the install.
This is considering you have the bottom end set to TDC and using the bottom end locking bolt from the teardown of the top end (which I did).
So at this point everything is off and the conrod is exposed ready for the new piston and crank is at TDC.
Mount your piston rings onto the new piston. I went with the Vertex OEM replacement high-compression "A" or "I" piston as KTM calls it.
I followed exactly the instructions from the Vertex website on installing rings.
Start with the oil ring (the squiggly one), put it on in the lower groove of the piston and make sure the ends do not over lap. Also put the gap to the side of the piston. Next put your bottom oil scraper ring on into the same groove as the oil ring just slip it underneath. Put this scraper ring's gap to the intake side of the piston about a centimeter or so off centre of the intake direction. Then install the top oil scraper rings into the same groove as well just sitting on top of the oil ring, and put the gap 180 opposite of the bottom oil scraper (just off centre from the exhaust side of the piston). Next mount the compression ring, make sure the letter marked on the ring is facing up and install in the upper groove of the piston.
Install the piston with rings into the cylinder while still off from the bike/con-rod. Make sure the triangle marking on the exhaust side of the piston faces the exhaust side of the cylinder. Using some assembly oil or 10w40 motor oil and a q-tip, oil all the rings on the piston, also oil the side pads so they are lubricated when you install in the cylinder (i even put a thin layer of oil on the cylinder wall). Flip the cylinder upside down on your clean work bench and install the piston from the bottom being very careful of the rings not to snag them. The end of the cylinder inner sleeve gives to rounded half circle cut outs which lets you use your fingers to compress the rings enough to slip it into the cylinder (this was surprisingly easy I though would be much more difficult but with the cut-outs it's a nice straight forward install). Try to push the piton evenly into the cylinder.
Once you are in and you slid it up and down just enough to feel that it slides nicely and rings are not grabbing then bring it down to it's lowest poition of being exposed from the bottom but not letting the rings get out. This way the hole for the wrist pin is accessible. While the cylinder and piston are still off the bike make sure to install one of the spring clips for the wrist pin and make sure its the one for the side of the piston on cam chain side. This is the side you have little access to when the cylinder goes back on the bike so you might as well install the spring clip while it's off the bike and you can get at it. Set the "hooked" end of the spring clip into the groove it has on the piston (notice one side is a sharp right angle and the other is more shallow and better follows the shape of the hook of the spring clip. slip it in this way). then hold and press with your finger tightly towards the groove in the piston where you are aiming to pop it in to seat it. With a nice firm and committed motion, try to do it in one shot "pop" it into the groove with a thin flathead screwdriver. Carefull not to stab your hand/fingers as i almost did. Once it's popped in make sure to check all around the spring clip to make sure evenly seated into the groove. Also, what I did to make sure its nicely in there is I lubed and installed the wrist pin and pressed against that side of the spring clip to make sure not moving and properly snapped into place. This one went in perfectly. Take the wrist pin back out so you can move on to installing the cylinder and piston onto the bike/con-rod.