I found a nice DIY video for this
The vid is for a Jag using the ZF 6HP26 transmission (which is a slightly bigger version of the 6HP19 in my E90 325i.) The process is quite similar to what we'd see on an E90, the exhaust pipe might not be in the way though.
He shows a nice trick to loosen that pesky fill plug. Also (surprise, surprise!) he uses Mercon SP ATF
I have crosschecked this on other forums (Audi, Jag, LandRover, Hyundai Genesis) where they have models with the 6sp ZF transmission. Mercon SP is indeed being used as a cheaper alternative to the ZF Lifeguard6. Hyundai actually charges around $75 for a quart of LG6
Anyway, to each his own. I have bought myself plenty of Mercon SP and plan to do a drain and fill pretty soon. HTH