Jim,
It is hard to explain the procedure, because the clearance is so tight. The stud that the bearings are mounted are a snug fit but not a press fit.
You have to slide the edge on the inner channel that the bearings are mounted to inside the outer channel, to get enough clearance to slide the new bearing into position. On the last one, there is only enough clearance to slide the bearing into position with the mounting stud inside it until you get the stud lined up with the hole that it mounts in.
Then you have to align the channels so that they are perfectly even and parallel, so that the slide will function smoothly. Unless you are at least a pretty fair mechanic with a lot of patience, you should consider having them replaced by someone. However, if you have the time and patience along with a little ability it is doable.
Gerald