To begin with, you will need the correct foil, solder, flux and soldering iron. In fact, you will also need a good work surface as well. We solder our windows on top of a sheet of particle board which we lay on top of our work bench. We prefer not to solder directly on top of our bench since flux will flow into and onto the work surface you will use. Not to mention that you can scorch the surface you are soldering on. We want to keep our work surface clean for making patterns, cutting our glass etc etc, for that reason we keep our table top flux free!
As far as using the correct foil, what we mean by that is, using clean foil. That means foil that doesn't have the backing glue on the outer copper surface or contaminated with other materials. If the foil is contaminated carefully clean it with either rubbing alcohol or Goof-Off. When foiling you need to foil the pieces with equal foil on the front and back side of the piece of glass. The foil needs to be flattened down with a fid (a flat plastic stick or even a pencil) to make a nice smooth surface for soldering. If the foiling is flawed or sloppy, the solder job will also look flawed since you can't add additional solder to an area that doesn't already have foil. So take your time when foiling and do a good job, or it will show later. When soldering lead came, the came needs to be clean too.
For solder we prefer to use 50/50 solder for lead came and 60/40 for copper foil.
With flux we like to use paste flux for lead came and liquid flux on copper foil. There are types of flux that seem to sputter when heated. We try to stay away from those brands of flux. They can actually become dangerous by allowing small particles of molten solder to become air born and perhaps land on skin or clothing.
We advise glass artists to use soldering irons made for stained glass. Some irons used for circuit boards do not heat up enough to work properly. Other irons only have small iron tips which can make it very difficult to solder easily.
Once all of the above conditions are met, it is time to start soldering. We start soldering on the front side of the window, once completed we flip the window over, solder the back side completely, then flip the window over to repair the front where solder may have flowed from the back to the front. We flip the window back and forth until we are happy with the soldering. If the window is to have a zinc frame, we solder it on at this time, as well.
On the front side of the window, when we begin to solder a piece, we like to tack solder all of our glass pieces into place. By that I mean, we like to put a little solder on every piece just to keep all pieces from moving and distorting the window's shape, as we solder. We do butt our window up to the outer area of our work surface where we nailed two strips of wood trim into a 90 degree angle (we do this to keep our window square). As seen in the picture below.
Photo from the Colson Stained Glass Studio
First apply the flux to the entire piece. We use a small paint brush which can be purchased at most stained glass retailers. When soldering you want the solder line to look rounded, not flat. If an area looks flat, add more solder so that the solder line is rounded! It is not necessary to solder in any specific manner, it is more important to have the correct finished look. If you end up with rough, dull colored soldered areas, that is indicative of cold solder. To repair an area like that, add a little more flux to the area and heat up the area with your soldering iron. You also don't want excessive blemishes and large clumps of solder. To repair, re-flux and re-solder. It is not necessary to always add more solder, it is possible to just heat the offending area with your iron and correct the issue.
When done soldering, both sides, wash off the flux. Flux can eventually deteriorate the solder if left on the window. We clean the flux off our windows by using a sponge with a small amount of dish detergent.
Finally, please watch the soldering video we previously posted SOLDERING COPPER FOIL. There are other soldering videos on You Tube and online which are excellent to watch and give hints and tips to new glass artists.