The way to bend larger leads such as flat outside leads around pieces of tracery or other curved shapes in window panels is to lay the lead upon the bench, and use a curved, preferably wooden, lathekin.
Progressively manipulate the lead into the curve. Hold the lead steady by keeping your fingers spread on the top and back of lead and manipulate the curve between your extended fingers. Gently push the curved lathekin along the heart of the lead with small, smooth, circular strokes. Smooth the lead flanges by pressing down on the flanges on the inside of the curve as you go. If you try to do it too quickly the lead will probably buckle.
Frequently turn the lead over, applying the process to both sides.
If the flange crimps or buckles, put smooth jawed pliers inside the lead and squash the flange flat. The pliers can be used to flatten any kinks that develop in the lead.
The key is to handle the lead gently and in stages, gently flattening the complete lead and not flattening completely one spot before moving on to the next.
The advantage of round over flat in this circumstance is that round came of the same size can be bent into smaller curves that the flat came of the same width.
The technique for finishing a curve around a single piece of glass can be seen in the tip “Leading Small Circles”