Literally, you put it between two posts, or in the crotch of a tree, and bend it a little at a time. You must be able to shoot it to get the pattern shooting where you are looking, but this is how it is done. Some gunsmiths use jigs to get more control over the process, but some do not. An old gunsmith in Savannah would lay two bags of lead shot on the floor of his shop, on top of each other, and holding the barrels by the muzzle, bring them down like chopping with an axe onto the bags in such a way as to get the bend he needed.
It sounds like shade tree stuff, but I have seen it done perfectly on a M12 Winchester barrel by wedging it in a crack in a shooting bench and carefully bending it. A couple or three tries and the gun's pattern moved over perfectly.
SRH
