Very quick guide to soldering
The soldering process can be summarized with these four keywords:

Before you start: practise this on scrap material first!
You will need:
  • A soldering blow torch. For small jobs a pencil-type propane torch is fine. For larger ones, you may want to invest in a blowtorch that’ll run off disposable propane/butane cartridges.
  • Solder. I tend to go for solid wire solder, by prference silver bearing and lead free. My favourite guage is 2.4 mm. This is standard plumbing solder, and available in any decent hardware store. Avoid very soft solders, as these may contain cadmium which is extremely poisonous.
  • Flux. My favourite is Fry Technologies Powerflow Flux paste, again available pretty much everywhere (in the UK, at least). Another favourite is Bakers Fluid No.3.
  • Sandpaper, wire wool, detergent – in soldering cleanliness is next to Godliness.
  • A fireproof surface – I use an old ovendish, with lots of different copper pipe offcuts and bits of scrap metal to keep the work in place. You can solder in a vise, but these tend to absorb a lot of the heat.

The first and foremost thing to check is that the join to be soldered is scrupulously clean. Make sure that any traces of old solder, dirt, grease and corrosion are well and truly gone. The surface of the join must be bright, clear metal – otherwise it just will not work.
Also ensure that the two surfaces to be joined are a good tight fit. The solder will be drawn into even the smallest gap by capillary action, but it is no good for filling. The easiest join to do is where the two parts are a tight sliding fit (for example, 1/8 pipe into 3/16 pipe)
Now make sure that the join has a good covering of flux on both sides. If you can rotate your join, do it, just to make sure everything is good and covered. The flux will prevent the metal from oxidising while it’s being heated.

Now heat the join VIGOUROUSLY – don’t be shy with your flame. If you bring the work up to temperature too slowly, the flux will just boil away. The flux will quickly boil, and run into the join. When you think the work’s hot enough, take your solder wire, dip the end in the flux paste, briefly avert your flame and touch the wire to your join – if the solder doesn’t run in immediately, the work’s not hot enough – apply more heat.

If after a few attempts it still doesn’t work, you’re not applying heat quickly enough. Start over from the beginning, and use more torch power this time.
If you’ve done everything right, the solder will immediately “flash” into and around the join. Apply a bit more, until just a little of the solder is visible around the join. Remove the heat, and let cool slowly. Don’t handle the work while it is cooling.

Wash the work in detergent to remove any flux traces. Remove any excess solder by scraping, sanding etc….don’t be tempted to “melt off” excess solder – it will just run into the grain of the metal, and be even harder to remove. Finally, polish your new join to parade gloss, sit down, pour yourself a drink and pat yourself on the back for a job well done!

Couple of considerations: This technique is called
soft soldering – it works well for plumbing applications. It’s also fine for steam applications on the boiler, steam pipes etc, as these don’t exceed 100C by all that much. For hotter applications, flametubes etc, or for higher pressure applications, silver soldering (or brazing) is required. The basic principles are as described above, only a solder with a very high silver content is used, at much higher temperature, with a much more aggressive flux. This is not something to be undertaken lightly, and if you are at all unsure, leave it to an expert.
Be sensible! Keep heat away from flammable materials,
don’t breathe in any soldering fumes (make sure you have adequate ventilation). Remember that all this stuff gets really, and I mean REALLY hot!

Hard soldering - very different, so I did this little video: