Before we go 25 feet into the air let's practice making the knots we are going to need up there. The first one is called a bowline, you can find a picture of it in any boy scout or sailing book. We will use it to tie the ropes to the metal loops hanging from the ceiling. Its beneficial properties are that it is a non-slipping knot that creates a fixed size loop in the rope.
The other knot is the secret Baker lighting knot. This knot is used to tie the rope onto the bars. It is good because it can be tied relatively quickly once you are good at it (a good thing when you or someone else's arms are getting very tired holding a heavy bar high up in the air). This knot also holds the bar in such a way that it is difficult for the bar to roll, thus preventing the bar from rotating and causing the lights to point in directions that you had not intended them to point. Finally the knot will also hold even if the bar is hanging free from one end, in other words it will suspend the bar in mid air for you while you reposition your ladder to tie the other end. So, you ask, how do I tie this wondrous knot? Imagine the bar horizontal, and the rope hanging down vertically. Loop the rope two turns around the bar, now bring the rope over to the other side of the rope hanging down from the ceiling (so that the rope you have in your hand goes around the one hanging from the ceiling in such a way to help hold the loop in place) and make another loop around the rope in the opposite direction as the first two (in other word if clockwise before you should do counterclockwise now). The remaining rope should be tied in a two half hitches (once again see a book on tying knots) to the rope hanging from the ceiling.
I apologize if the descriptions of these knots is hard to understand, but this is really something that is better demonstrated by someone that already knows them, rather than trying to understand it from reading a multi-paragraph description.