1. Loosen the bolts at the flange that connects the exhaust to the header, and also loosen the exhaust hangar where it bolts to the muffler.
2. Place a floor jack and block of wood under the muffler and raise your jack to level the muffler and hold it in position.
3. Tighten the bolts you loosened on the exhaust pipe and header flange, and on the muffler hangar. Note that the muffler hangar has some adjustability in the slotted hole on the muffler.
This is the procedure I followed and it got my mufflers pretty close; not perfect, but pretty close. If that doesn't work, then I think you might need to drive by your local "trusted" muffler shop for adjustment.