Basically I'm now on an amateur team that needs some rocks and trees, and everything I've done much modeling for up to this point has been explicitly man-made, with the exception of a small handful of unfinished humanoid characters. I do know ZBrush a bit, but the geometry that would go into that program is important too and I'm not totally sure how best to make it all. Just some "things to definitely not do" would help. Thanks.
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There are some great tutorials and videos around that show from start to finish. Start making your way through them, and if you run into any problems, come on back with some more specific questions and you'll get some better replies. Good luck!
If you plan to bake normals, you can take it into ZBrush and divide it (disable smoothing) a number of times to add density then just gently sculpt details. Once you're happy with that you can either generate the normals within ZBrush or take it into siomething Like Maya which has transfer maps.
Goodluck!
Google is your friend