Professional sheet metal workers can make long straight bends in sheet metal by the use of a machine called a 'break'. One part of the machine clamps down on the sheet while the second lifts the rest of the sheet up to some angle. A home made break can be as simple as a 2x4 clamped down over the sheet metal, or a jig like in the first photo above where the 2x4 is spaced just off the plywood base to allow sliding the metal in and around. The lifting part is just a piece of plywood, either hinged onto the base, or free as in the photo. You can buy rolls of sheet metal for flashing work around the house that are one to two feet wide and about as long as you might want.
The second photo shows a typical application of a simple flashing, a drip cap over a window, that is easily made with a home made 'break' -- and keeps the window from leaking. The common error with drip caps is not maintaining the slope you see in the photo, which prevents water from puddling on the ledge. With a home made break you can make this flashing just the right size to fit snugly over the frame and maintain that water shedding slope.