Hopefully this will help and keep you from wasting a bunch of diamond plate.I made a couple of pieces for the lower rear corners of my XJ and bent them using a bench vise, a hand few small c-clamps, and a bunch of scrap steel.
First and foremost, 1/4" aluminum doesn't bend as easily as you'd think. At this thickness it will have a tendency to crack on the outside portion of the bend. 1/8 or 3/16" might be a better option for a DIY job.
Second, layout all of your bend lines before you start bending anything, allow an additional 1/8" for the bend radius between any two bends.
Third, use four pieces of steel clamped above and below the bend lines and on the front and back of the diamond plate.
Fourth, if at all possible, make sure your bends are in between the diamond portions of the plate. Trying to bend accross the diamonds by hand will lead to a really lousy bend.
Fifth, slow steady pressure should be applied. Use a cardboard template that matches the surface that the diamond plate will be attached too. Don't over bend the aluminum, it's easier to go back and bend it a little more than it is to try and straighten it out if you bend it too much.