This is a multi-step process but it works:
1. Buy any 'new work' can. They have bars on the sides that allow you to position the can when you nail it into wood framing. Every big box store sells these cans as they are the most common type. Also get some small 3/8'' sheet metal screws that will fit in the holes next to the nails in the bars (cuz nails aren't going to work so great on a suspended ceiling).
2. Go home and position the lights where you want them and use the spreader bars to position them. DO not screw anything together yet. Now wire all the cans. Secure the wire to the wood ceiling joists. You should leave yourself a 1 + or - foot piece of wire from between where it is secured with a staple to the wood down to the can. Do not run the wire directly over the cans, do not position the wire in such a way that it may come in contact with the can.
3. Next, take a ceiling panel and mark on the back the exact center of where the can will be. Take a nail, stick it thru the tile, and using the included tracing template (make sure at least one of your cans has this stuffed in it when you buy them) trace onto the FRONT side of a panel exactly where you want the light using the nail as the center point. Now use a sharp razor knife to cut out a hole in the tile for the can. (You can cut from the back side of the panels, but I find it comes out neater it you cut from the front).
4.Put the panel in the ceiling, and position the can over the hole. They usually sit up there nicely without any adjusting for height. Now use the sheet metal screws to secure the spreader bars to the sides of the ceiling grid in the holes next to the pre installed nails for the cans. (You can leave the nails, or not, it doesn't matter)
5. Repeat steps 3 and 4 until your done.
A couple of notes, 1. You cannot remove the ceiling panel after you have fastened the can to the track, so plan accordingly. 2. Getting the sheet metal screws to start into the track can be a pain. If you are unlucky you may have to use a marker to mark the holes, move the can and the ceiling tile, pre drill the holes, then put everything back and screw the spreader bars to the track using the pre drilled holes. And this probably goes without saying but keep your dirty hands off of the white ceiling tile! - Queue Mission Impossible soundtrack.
-rick