Collar material should be thinner than the metal being joined. For joining a pair of 1/4 by 1/2 scrolls, a 3/16” x 1/2” collar stock works well. If you make it out of 1/4” thick stock, it will look too massive.
The collar length is the circumference, 2” in this case, plus twice the thickness, for a total of 2 3/8”. Hot cut part-way through, leaving a handle that can be snapped off later.
Stamp the first bending mark just shy of 1” from the end. (To-do, figure out the math for this.)
First Bend. Place the bending mark just above the mandrel in the vise. Putting the stock on the back side helps to see the mark, which will then be hidden inside the collar.
Second Bend. Turn the mandrel long so that the first bend can’t strike the vise. Bend towards yourself again to form the U. No marks are needed for this bend.
Third Bend. This uses the step on the anvil. Bend the short side first towards the anvil’s face.
Fourth Bend. Walk around the anvil so the long side is near the horn. Snap off the excess, then bend towards the anvil’s face.
Final prep. Heat the back of the collar and open with pliers. The back should bend, leaving the corners intact. If the corner bends close it back up and try again. (You can’t fix the corners once it is on the scrolls.)