I hired a plumber who I think was drunk when he showed up at my house (6PM) and blabbed all his secrets to me. But, man I learned more than I think I could've from a book.
Here's a summary, then I'll follow up with the details/explanations of each step.
General tips
1.) Debur all cuts
2.) Do not touch any copper part you've sanded until after you solder.
3.) Minimize the number of solders you do in location. That is, do as much pipe and soldering as you can at the workbench then put in place.
4.) Sand or wire brush both the pipe & fittings and 1/4" - 1/2" more on the pipe.
5.) Try to avoid doing half a joint, then later reheating and trying to solder the other half. This contradicts rule #3. I've had far more success doing half a joint at the workbench, putting it in place and doing the other half than putting it in place and doing both sides at location so I recommend breaking this rule if at your bench.
6.) Solder flows from cold to hot. Go side to side, and around as much as you can with flame but heat mostly the middle of the fitting. Apply the solder to the coldest side (the side you hit the least with the flame) it will suck it to the hot side since it flows towards the hottest, and be sucked to the middle of the joint since that's hottest too.
Application
1.) Sand the pipe & fitting, and the pipe needs 1/4" - 1/2" more than will go into the pipe.
2.) Apply a coating of flux, don't over do it. Make sure you also go a little past the fitting.
3.) Apply the flame back & forth to the fitting but mostly the middle of the fitting. You know when it's ready when the flame turns a solid green when near the end of the fitting. Don't overdo the flame or you'll burn away the flux... hit it as soon as it will allow (which is usually a few seconds after the flame turns green).
4.) Apply solder to the cold side of the joint (the part you hit least with the flame) and go around the edge to make sure it goes in. They say 1/2" solder for 1/2" pipe. Using surface area that would mean 1" of solder for 3/4" pipe and 1 3/4" solder for 1" pipe.
5.) Let the pipe cool without touching it with a wet cloth.
6.) Follow up by cleaning the joint with a wet cloth when it's cooled a little.
More detail
1.) Others have discussed the benefits of debur.
2.) The reason you don't touch any part of the copper pipe you sanded/cleaned until after soldering is the oils in your skin can interfere and cause the joint to fail.
3.) You need to clean 1/4" - 1/2" past the fitting and flux it as I have a round wire brush thing from the big box stores that's 3/4" deep for a 3/4" fitting (and sealed at one end so I can't slide it more onto the pipe). I've used this enough times it just cleans where the fitting goes and nothing more and when I go to solder, the solder hits unclean pipe (doesn't matter if fluxed) and bounces off it like oil & water instead of melting and being sucked in. I would rate not cleaning/sanding 1/4" - 1/2" beyond the fitting (if it's a 1/2" fitting I sand 3/4") and putting some flux on the part beyond the fitting as the #1 reason for my failures.
4.) Flux is bad stuff, that's why you don't want to gob it.
5.) Solder flows towards the hottest part, that's why you want to mostly
heat the middle of the fitting and apply the solder to the cold side of the fitting when ready.
6.) Let the pipe cool naturally because metals expand/contract at different rates and throwing a cold towel on the hot copper fitting, the copper will shrink at a much different rate than the solder and you may ruin/crack the solder/joint doing that causing a failure. You do need to wipe it afterward as flux is bad stuff and you'll usually start to see green deterioration if you leave it but I'm not of the school that hitting it with a cold towel immediately, knowing metals expand/contract at different rates and shocking the joint can possibly be good for it.
Good luck!
