I do the rust by first applying a 3D texture medium, dobbing it with the tip of a bristle brush. Once it's dry, I spray it with Testor's Dullcote, and then apply several layers of chalk pastels in a few different shades of rust, sealing between each color. Then I go over that by wetting a q-tip and dobbing on brown pastel with it. Once that's done, I finish it off by taking a flat brush with just a little antique gold paint on it and gently brushing over the top of the rust, which really pulls it all together and makes the texture pop.
That's about it. It's not difficult at all, it just takes a lot of steps.