It is not as simple as marking and cutting. The initial cut is simply that, but I cut it a little lower the actual line will be. I use a nibbler where I can to prevent distortion from heat. I make a jig to get a reference for the bottom of the corner panel. I then remove as much metal as needed to get the proper fit and gap to weld. The little clamps are nifty, but they are not a fix all. The patch panels are not always the same contour as as the parent metal. You will want to weld in small stitches to prevent warping. Quite often you will have to bend the metal a little between the stitches to be a proper match. Time you spend aligning the two pieces will greatly reduce the amount of grinding later on. Practice a lot on some junk first. Weld 2 curved pieces together and then grind and then lay a something over the splice with the same curvature. With practice and patience, you can make a patch you can be proud of.
