The best way to make a good impression is to be on time, be interested, read when you have time, and demonstrate that you are learning.
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That, and don't complain or act grossed out or ask if you can leave yet.
One would perhaps be appalled at how many residents can't cut and examine a heart adequately or cut and lay out a fresh brain. Forensic pathologists all generally have their quirks, so if you're working with multiple different attendings then be sure to take the time to ask each day what they would like (at least if they allow you to cut, which most of the time they will, but it depends on local issues or the day's cases). Like a student rotation, there usually isn't a lot of high expectations even for pathology residents rotating through forensic path, because they know you don't get much exposure specifically to forensics. But if you tell them you've already done 30+ hospital autopsies they may expect you know the basics of cutting organs, if not necessarily the interpretations they may apply. It's also true that different people eviscerate differently -- I've used both "common" methods (removal en bloc vs organ-by-organ) during residency and in forensic offices, so they're not specific to one system per se. If what they do is new to you, just say so.
Beyond that, during the rotation work on your efficiency. There is usually not a lot of "wasted" motion in forensic pathology, and when there is it typically has a particular purpose (preparing to answer an expected question for family or a lawyer, even if medically you don't think it would be relevant), and speed-without-missing-something becomes a marker.
Edited to add: I didn't notice you're listed as a medical student; I assumed 3rd year meant resident. But that's OK too. If you've never been involved in an autopsy before, say so, and they'll probably have you watch for a few days then, depending on local laws/protocols, may let you cut. Ways to "stand out" in a good way still remain pretty much the same. Show interest and attentiveness, tell them if you are/aren't interested in pathology/forensics and they'll often tailor what they teach you (in a good way, IMO), but try not to be so bubbly that you're annoying and in the way.
