I had forgotten the techniques for using a gas torch and had to experiment. After a few failures I watched some YouTube videos.
The bit of information I needed was the flux. As far as I could see, everyone uses a sticky paste flux.
I used the same flux cored solder I use for electrical work.
I also had to buy a new torch because the pen style torch I was originally using started to leak. I ordered a posh looking but very low cost butane torch. Easy to use but a bit messy to fill. I like the piezo self igniter and the push and hold button to use.
It has a lock to hold on the flame, if necessary but for soldering small joints you only need the flame for a few seconds so I didn't use the lock. There are separate gas and air flow adjusters so I could get the flame exactly as I wanted, about 10mm to 15mm (0.5") long to the centre blue flame point but it would reach over 50mm (2") if necessary.
While looking at the YouTube videos I came across other tips. Cleaning the joint area with sand paper, I already knew. One suggestion I tried was cutting a small bit of solder and laying it on the joint, stuck in the flux. That was handy on a few joints but with some practice I found it unnecessary.
My end technique was to apply the heat to the joint. Move the flame away momentarily to add the solder, move the flame back to melt the solder, wait until it flows then remove the heat. Ideally the heat should be applied on the opposite side to where the solder is to be put on but sometimes that was not possible.
The next issue was supporting the joints while soldering. That can be tricky but pretty much anything that won't catch fire can be used to keep the bits of metal together prior to applying the solder.
Being so tiny I also had a few cases where heating up one joint melted the solder on a previously finished joint. Frustrating but using a pair of tweezers as a heat sink usually solved that problem.
The model, shown in the photos for the roll cage, is a failed 3D print I had used for practising smoothing the surface. As you can see, that was handy to offer up the metalwork without worrying about damaging the paint on the work in progress.
The last job was to tidy up the joints. That was done by filing and sanding away the excess solder.
Where to buy?
If you plan to buy any of the things mentioned on this page, here's a few places to start looking:
The less poisonous flux I am using
Micro gas torch