I completed my first season as a river guide last summer and it was awesome! The best training I think was just running the section of river we guided on over and over and over again. After some practice runs with the head guide for the school we had to log-in a total of 20 guided runs down that stretch of river before we were allowed to test out. If you can read water you've got a large part of it down already. One thing you could do is learn about your solo paddle (canoe) strokes that you would use as the guide in the back of the raft: we called these the J-stroke, sweep, pry, and draw, and these were the four we needed to know. There are a number of YouTube videos on those, for example:
For the strokes I practiced using a canoe paddle in an IK, and then started using relying on them R1 in a small raft. When boating with a group of guides you can practice your strokes in calmer water until you get the hang of them. You do need to mind the weight of your boat and the current when trying to solo-pilot a raft full of folks, but you begin to get the hang of it! I remember my very first time trying to move a boat in current using solo strokes and wow was it frustrating...but it grows on you, it will come!
What river are you thinking of guiding on? I was guiding the Rio Grande (northern New Mexico).