This is not the only way, -but it is the method that works for us. We have a 1x12" board that has the radius that replicates the back section on each side. Remove the gutter from the back piece and insert wax paper or Vaseline so the two pieces do not bind or gall when making the curve. Slowly (i.e: SLOWLY) start forming the assembly around the end while constantly checking the fit. Most of the time, the repro gutter assembly is longer than what is needed, so move forward enough where you have excess on the bent end to trim.
With regard to annealing, these are pretty soft to begin with, so try that first. If you find yourself needing to soften it a tad more, be aware that the best way is to use an Oxy/Acetylene torch (-not a little propane torch). To begin annealing, you will use an Oxygen-rich mixture (-sans the Oxygen) to soot the entire area. Then add a little Oxygen to clean-up the flame where it is a soft blue, Then apply only enough heat in a fanning motion where the soot is burned away from the aluminum. Once cooled, it will be annealed.
If none of this is clear enough, let me know and I will see if I have any pix to send. You might also check in the older posts because we have discussed this some before and I may have posted pictures then.
