If it's huge and all crinckled up, drill little holes in the deepest part. Use a dent puller. Knock down the high spots with a hammer that are higher than the original surface. Try to sand off the paint so the bondo will stick better. Go buy you a can of the old Bondo. Bondo away. Sand, bondo, sand, etc. until results look good. They make some orange stuff to fill in the little holes and blemishes in the bondo. Takes a lot of practice to make it look good. Prime, and paint. Good luck. I think the procedure is in any Haynes auto repair manual.