I had an eisenmann on my E46, install is about the same, if you are doing it yourself.
Get the car in the air (ramps are best), and detach the whole exhaust system, right off the cat/header. (4 bolts). Disconnect all the other brackets, etc etc and remove the whole exhaust system. Compare the eisenmanns connection point to the OEM, and cut the OEM with a sawsalll, or a hacksaw. Measure twice, cut once!
Reinstall the OEM pipes etc. Then install the eisenmann onto it.
Note: for 100 bucks a muffler shop will cut your stock exhaust off and weld on the eisenmann, which is a lot less hassle, and you will have a better connection; the clamps tend to leak.