Here a video how to do on the E36, made by jbate (M3 C) and his comment:
This video will give you an idea of what you have to do while the handle is still in the door. First you need to pull the rubber cover back. I have seen a DIY where the guy used a paint gallon lid opener, I used a set of picks. The whole thing is done by feel because you can't see a damn thing.

The gaskets I sell here under Swap Meet - Classified Ads
outer door handle gaskets E34, E32, E36, Z3

4-door set or also 2-door set available.