The rod guide should slide through the hole on the horn. If it doesn't, you may need to widen the hole a little bit. The best tool for that is a drill bit the size of the post. An exacto blade can auger it out, but it may be a bit sloppy, as the hole will be beveled. You can also heat up another object just under the diameter of the guide post and slide it through to melt the hole bigger. If you use the post, the hole will melt and expand past a snug fit, and you'll get slop.
Which hole you use is personal preference. The innermost hole will give you the greatest control surface throw, the outermost the least. If you're using a computerized radio, you can use the closest hole on all of them and then dial the rates back on the radio - that way you can "grow" into the plane and won't have to reconfigure the rods. If using the stock radio, then I would use the innermost or middle hole on the ailerons and rudder, but the middle or outside one on the elevator. Mine was crazy on pitch, and that "helped" me crash my first few flights. It just depends on how daring you are, and how docile (or not) you want the plane to be.
If by "backing plates" you mean the small washer-like devices that clip onto the post of the rod guide, then they slip on. You may need some small pliers or a tiny nutdriver to push them on. Once on, they're not easy to get off, and we like it that way!
Welcome to the hobby, and the forum. Good luck!
Yes, I've known that I'm "different" for some time now...