We all have some frame rust, some more than others. How do you go about starting to fix it? I will be wire-wheeling the rust and then painting over with a simple primer-paint. Do you select several of the worst welds/sections around the frame and get those fixed first, in patches? Do you pick a section, getting everything nearby repaired, and then move on to another small section?

I have rust scattered everywhere, but I don't think it is too bad. The picture is just an example, not necessarily the worst on my truck.

The best way, but not the easiest way is to remove the rear axle. With the axle out you'll have better access to all areas of the frame.

Either way you want to clean off all the dirt, rust, and loose paint. Then clean every down with Acetone. After that I'd get a rust converter on it, then a couple good codes of primer, and then your paint of choice. Chassis paint works good, or even a bedliner. The bedliner will be more difficult to clean, but it's more durable. You can go a step further and apply Fluid Film, and that stuff work great.

I consider myself a rust "veteran" with my last truck. In my experience, the thinner metal rusts first. All the brackets ( i.e. E brake brackets, O2 sensor wiring brackets, the inner frame rail walls, the brake backing plates) rust the fastest. I also noticed how bad my fuel lines were rusting when I dropped my tank. Also, Some of the crossmembers start rust from the inside. I had to replace the one crossmember that the transmission mounts to.
As mentioned, to really rust proof any of these things they need to be removed and have your rustproof of choice applied to ALL bare metal. I used KBS Coatings which is similar to POR 15 but there are others. Two things I was going to buy before that truck got totaled was a camera inspection tool and a sand blasting pot. I have access to a cabinet sandblaster and that's the best way to get all rust off before you rustproof the part. Once you get is all coated then I would use a Fluid Film type spray to add a final layer of protection and reapply yearly.