Here's a video, and there are a few others, just search for "BMW side vent"
Full disclosure, I did this and it was a pain in the ass. You will very likely break at least one clip. Might not be much of an issue if you're replacing the originals though.
Some tips - I ensured the area around the side vent was well masked up to protect the paint. I used multiple trim removal tools, starting from the door side working towards the front bumper. I inserted the first one and began to pry it out, then left it in place and used another trim tool further along the vent to keep prying. Work your way around and it'll become loose enough to pull out.