So what if you have all that experience with mammals and fish, still makes you a BEGINNER for reptiles. I don't do fish even though I have national awards for my mammals and reptiles. That makes me a beginner for fish if I would get started on them. I sure would not do the very first fish attempt for a Nature Center or any client for that matter. But I guess, since they know you have not done any snakes, and that's why they gave you the job

they don't expect much.
Ben Haden has a rattlesnake mounting video you may check out.