what i did was clamped it down with the tip part upwards. Used a demel w/ cutting bit to make slots (maybe 7 slots), then put the dremel tool into the hole and started to cut upwards. Each of the pieces were cut off and use a grinding bit to ground down the sides so that it is level with the sidewall. Then i bead blasted the inside to remove any carbon or marks from the bit to make it look cleaner. Used mother aluminum/mag polish to make it look pretty. its kinda hard to explain, i wish i had pictures.
and yes i did cut the muffler shorter by about 3 inches, has a different sound also!