I have done a LOT of rust body repair, specifically floors. I got my experience on a '70 mercedes coupe so it was all trial and error. What I found to be helpful is get cardboard about the thickness of cereal box and use that to cut to shape your patch, it is very easy to cut and you just keep trying and re-shaping until it is perfect. Mark on the cardboard with sharpie how you want your bends to be. Then when the cardboard is perfect, it is easy to trace the shape onto sheet metal and cut that out; chances are you'll need to fine-tune that a tad but you'll be very close. I repaired the lower portion of my driver's side fender in this method on my '91 pathy just the other day.
Here's a tip, the absolute BEST rust paint out there is called "Zero Rust", it was designed for painting bridges. It is pretty expesive and REALLY stinks, but it is bullet proof and will stand up to absolutely anything. I've used this stuff a lot and it has never let me down, it is perfect for rust repair work and chassis painting.