I take a screw driver, and carefully work my way around the crimp in the muffler, prying it open. The muffler can then be taken apart. After gutting the muffler, I then put JB Weld or silicone on the crimp, and crimp the muffler back together with pliers. Just be sure to leave whatever is in the muffler to prevent it from collapsing when the mounting screws are tightened. Sometimes it is two tubes, sometimes it is a squarish piece with holes in it. If that piece would be restrictive to exhaust flow, I either modifiy it, or leave it out and make up tubes to use in it's place.

AV8TOR