Soldering copper pipe is actually a simple and easy process, and can be error free if you follow so tried and true tips and trick:
1) Measure right
Be sure to measure the length of pipe needed to fit inside the joints whether it be elbows, t's, or other connectors.
2) Cut with a pipe cutter
Resist to urge to use a hack saw! Using the right pipe cutter will give a straight, clean, and burr-free cut. I like the small cutters as they are very versatile - they can be used in small spaces!
3) Clean the copper
The key to perfect solder joints is completely bare copper. Use a piece of emery paper or a cleaning tool and clean ALL the surfaces that will be soldered. This includes both the outside of the pipe and the inside of the connector piece.
Always do the finger check to make sure the area just cleaned has all the dirt removed.
4) Dry fit
Always, always dry fit your entire section of copper plumbing before soldering. This way you can find any mistakes in measuring. Plus you can plan the order of the joints to be soldered.
And last, but most important, mark the positions of the joints with a permanent marker - put a line on both parts - and long enough that subsequent steps won't obscure the mark. Often you have to take the assembly a part and solder it in sections, having it marked will make this process much easier.
5) Use a soldering flux
Soldering flux makes soldering copper a dream. It further cleans and prepares the surfaces to be soldered. The flux helps to literally suck the solder into the joint.
6) Protect the area
You are about to use a very hot flame to heat up the copper pipe, and unfortunately much of the surrounding area - studs, floor, wires, vapour barrier, etc... You simply must use a small sheet of metal to protect the surrounding area. Maybe the pros can do it without protection, but better safe than sorry.
7) Protect yourself
Things get hot when you solder, and you will if only absentmindedly reach out and want to hold a piece of copper that you just heated. The are all sorts of reasons to wear gloves - just do it. Eye protection is another big one. Melted solder can sputter - don't be dumb use eye protection!
8) Fire extinguisher
Accidents happen! Even with the best planning and protection a fire could start. Be sure to have a fire extinguisher within reach - not in the building - actually within a step or two of where you are soldering.
9) Heat evenly but focused
It is important to ensure the entire joint to be soldered gets to the right temperature. Often you will be soldering copper to brass fittings. Brass heats up much slower than copper so focus most of the flame on the brass parts.
But how do you know when it is hot enough. I am not a pro but I have notices that when the flame around the coper changes to a green colour I get the best joints. The instant it changes I remove the flame and touch the joint with the solder. The solder flows in to the joint perfectly every time!
10) Go Lead Free
With all the new products on the market there is no reason to use ANY solder with lead in it. This includes the solder flux or paste. I found some old solder in my tool box that had lead - resisted the urge to use it and off to the hazard waste depot it went!
Don't be afraid..
There is no mystic about soldering copper plumbing. Start with a few small joints, get your wings and then you will be soldering like a pro...