While there are a few ways to mark out lines on parts with a radius like a laser, drive belt, straight edge or a string. Sometimes the easiest way is with masking tape. With varying widths available you can pretty much manipulate it any which way you need.
In this case the fender needed to be narrowed a few inches as well as shortened and to add mounting brackets. A cutoff wheel on a hand grinder was used to cut out the strip, cleaned up and welded back together.
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