Copper is not a cheap material, so a job like this always starts with railroad board templates. Making a template is an interesting process. It is a nice combination of calculated thought and dumb luck, mixed in with a fair amount of persistance. I like to make one piece that represents the flat copper with creases where it should be folded (right side of photo). Then I make a second piece with the profile (bottom of photo). Together, these two pieces of paper give me all the information needed to confidently cut, mark, and bend the copper.
The next task is to miter the two halves. I start with a simple wooden frame that matches my window opening. This gives me the angles I need, and something to support the copper while soldering. It can be a bit of a challenge to mark the mitered cut across all faces of the profile. When available, I the laser attachment included on some miter saws. It is as simple as setting the desired miter angle on the saw, then tracing the laser line with a marker onto the copper. Now the only thing left is to find the right combination of snips to make each section of the cut. I routinely cycle through at least five different pairs.
Here is the finished miter on one piece. The mating piece has the same angle, but must include tabs to aid in assemble and soldering.
Here it is, ready for solder. For some reason, this piece did not want to take the solder. I tried many combinations of flux and solder, but it would not flow until I used the oldest tin of flux and the crustiest looking roll of solder in the shop. Go figure.
The solder joint is a bit messy and will need some touchup when I get to the second half.
Now that the lower half fits, I start the process all over again for the upper half. It must be a slightly different profile to fit up under the overlapping siding. When it is all finished, this window will get a stained glass panel.