A few boxcar upgrades...

[Sumpter250]

Thanks to all for the kind words. a 25 watt pencil-type iron. cleaning the step with 1200 grit wet/dry sandpaper. The steps were pinned to a sheet of 1/4" balsa, using dressmakers pins. After pinning, I applied a minute amount of resin flux to each joint, then touched one joint on each step with the "wet" iron. I then placed a small wad of wet tissue over each soldered joint, and repeated the process.
For small numbers of assemblies, the balsa-wood-with-pins option is an easy way to hold parts while they're being soldered. For jobs where lots of similar assemblies are required, though, it makes sense to build a more permanent jig of some sort. The ones used for soldering are made from hardwood blocks, glued together and drilled as required, while bending jigs can be wood, metal, or plastic, depending on the job and the material being used.
Wayne

It is amazing what can be worked out with a "pencil"...(the full tutorial is in Wayne's post above)
The trick here is the use of flux to transfer heat to the piece being soldered, and the use of water (in the tissue wad) to act as a heat sink to keep the solder in that joint from melting as the next joint is soldered.
The designing, and building of assembly jigs, is another art altogether, and one that is worth looking in to. Very useful things, jigs.
Thank you for that response Wayne.