Hi, I am in the same boat as you (first-time builder, solo riveter due to pandemic), and worked through section 26 in April-May. This is where you learn to back-rivet if, like me, you avoided it to this point. You will need a 12-inch back rivet set to reach many of the rivets. Some of the builder sites have good photos of the process.
This was a slow and painstaking process, at least for me, but I learned a new riveting skill that is useful for a lot of the lower fuselage and in general.
From what I learned:
Always (always!) ensure the manufactured head of the rivet you are setting is fully flush with the back riveting plate. There is a subtle curvature to the lower skin, so you will have to rebrace things every few rivets. A set of flat feeler gauges is helpful where you can use them.
Initially, at least, set the rivets incrementally, since it's a lot easier to drill them out if they start capsizing (because the rivet set wasn't straight) or setting proud (because the skin was not flush with the plate).
I used two workbenches in order to reposition the piece over the gap as required to allow drill-outs.
Don't mean to bombard you with detail, I still have a bit of PTSD from the experience.