1) Raise car, support on jackstands or ramps.
2) Unbolt the stock axle-back.
3) Bolt on replacement stock axle-back.
4) Drop car back down.

Don't use an aftermarket "performance" muffler with the stock pipes. It will look like hell, sound like hell, and you won't gain anything from it... plus, you'd have to weld it on. "Performance" axle-backs are just as useless. If you want it to make a difference, you'll need an entire cat-back (and an intake, header, and test pipe or high flow cat.)