Yeah Brians idea with the jig is a good one. If you don't make some kind of jig then you will have different looking fenders on both sides and picky people will notice it and never stop giving you hell about it. There is another way to do this too but probably way too hard to type it out here.
Get a book called "Metal Fabricator's Handbook" by Ron and Sue Fournier. Probably one of the best books I have bought so far to teach you the basics of making your own metal forming tools, buying them at a good price, making jigs to make COMPLETE front end grilles, roll cages, chassis, exhaust and headers, gas tanks and way more. Check this out and it should help you so you can feel more comfortable cutting up your fenders. www.Amazon.com has the book and it's where I got mine from. Fournier makes more books and they are good as well I am pretty sure because the guy is one of the best metal fabricators ever next to a few others.