One of my dyeing friends asked about rusting. I will try to explain the method I use. It is by no means the only method, but it has been successful for me.
First a word of warning. Dry rust can cause problems for some people. It can mess up your hemoglobin if you handle it to much with your bare hands. Be safe, wear gloves when handling rusty objects. Wet pieces do not seem to cause this problem, but play it safe anyway.
I usually start by laying down a piece of plastic. I place a piece of fabric that has been well-soaked in vinegar on the plastic and arrange my pieces of metal on top in a design that pleases me. Then I take a spray bottle of water and spritz over the entire piece to thoroughly wet the metal. You can either place another piece of vinegar soaked fabric on top or just lay another piece of plastic over your design. The fabric will pick up a mirrored image of your design if you place a little weight on top. The top of the pile is always another piece of plastic. This piece of plastic is there to slow the evaporation of the moisture in the pile. Air must be able to enter in order for rust to form, so do not try to make it airtight. Leave for 24 hrs, then remove the plastic and check your design. If it needs more moisture, spritz again with water and recover for a few more hours.