my recomendation would be to go to www.smooth-on.com
There are tutorials there on how to do body castings along with a listing of the materials you would need for the project. What I would recomend would be to do seperate castings of both breasts, then make a casting of the imperfect one. using say a cardboard box, cut out a hole that the mold of the perfect breast could fit into laying down, then you would pour your silicone casting material into the mold, place the casting of the imperfect breast (preplaning to make sure it rests level with the other mold and not sinks all the way into it and making sure there is adequate amount of mold release on both the casting and mold) the space inbetween the casting and mold will make the prosthetic to fit over the imperfect breast. Follow all instruction on preperation and curing time and make sure to powder the piece as you remove it to make sure it does not stick to itslelf. Check the site, I think there may be other options for hold than spirit gum. This is a basic idea, I do sculpting Since this is more of a personal nature and not making a sculpture for your living room if you would like help with this project I would be more than willing to do any work for free as long as you covered the cost of materials needed.