download cone layout from here
http://www.pulserate.com/
make a cone template the size of the exhaust pipe one end and the size of the 4 pipes the other end.
transfer the pattern on to the sheet metal. beat it round using a mallet with something solid underneath. once its round in the shape of a cone tack it to the 4 outlets then use a bolster chisel to shape it into the dips between each pipe then finally weld on the single outlet pipe.
dont forget to weld between the 4 outlet pipes before fitting the cone or the gasses will come out the middle between the pipes.
hope this helps its very hard to explain. but quite easy to show someone. i will see if i can find a video if how to do it after dinner.