Warning: If you are not competent or do not feel comfortable doing any of these modifications or procedures, then please do not do so. I will not be held responsible for any damages caused by a result of your actions. Now on to the fun stuff!

These are a set of original BMW tips that I purchased from the dealer.

Part Number for the 328i or 328is is 82-12-9-404-730, these will not work on a 325 muffler.
They slide on, then just tap them into place with a rubber mallet, and tighten the set screw.

Here are some pictures, sorry they're so dirty.