A good bed of coals and some forced air will get the tubing red hot but don't bother. If you heat it up and try to bend it the tubing will just collapse and kink. The only way that you're going to do any good is to get a bunch of 90's and 45's from the parts store and that will only work if it's a rather simple system to make.
For more complex systems you'll need a bunch of universal bends or sometime the header companies will sell you a box of header primary tubes. these are usually bending errors. Then you cut them as needed and tack together to make your bends,then add straight tube where needed. You will soon find out just how cheap most muffler shops are.