Hi Ron,
Preparation Is the first important step.
Very Important is that the surfaces you're going to connect are clean.
Rest old solder must be removed as much as you can.
You do this with a mini blow torch, as soon as the old tin becomes liquid wipe soft over it with a 000 steel wool, away from the boiler part.
Repeat this until you got nice smooth thin tin film left
Then go over it wit a soft nr.1000 water dipped sandpaper.
But make sure you only sand the seem, not the rest of the boiler.
I tape the rest of, so I don't have to worry while sanding.
If you have a well fitting lid, you must be able to push the lid over the boiler pipe, It not may fall of even if you shake it slightly.
Also cheque the seem between the lid an boiler pipe, there may be no gap, except a tiny gap the where the boiler pipe seem connects to the lid.
I use liquid solder (the kind they use for pluming), I lay a thick creamy layer on the inside of the lid, the corner age.
I clamp the boiler vertically in with the lid hanging or facing down, make sure the boiler seem is faced towards you.
It's of course important that you hold the inside of the lid face up not that the liquid starts running all over the place.
Work clean that's all I can say.
You also need very fine 1mm solder tin at hand.
Very slowly start to heat, in a round motion, with a mini torch, the underside of the lid.
Make sure you're doing this in a well ventilated aria, I don't have to tel you that those fumes are bad for your health.
If all goes well you'll see the solder filling in the gap between lid and pipe, except on the boiler pipe seem crossing.
Take away your torch, the metal is still hot enough to melt tin, so you can take that 1mm tin and melting filling up that smal gap.
Let the seem go cold naturally, do not put it into water.
As to make sure you have a tight connection.
You can seal off all opening except one where you can connect an air hose.
Put the boiler under 1.5BAR pressure, and dip it under water, if you see air bubbles you know that it's not sealed.
But to be honest and with my experience, I never do this, I you prepare well, you don't have to worry.
Important this is what I call a mini torch, and I think next to my lathe, this is one of the most used tools in the repair shop:
Should you have more questions a bout the tools or materials, let me know.
Cheers,
Danny