Hi — it’s actually very easy. When you have a surface file loaded in BrainSuite, you can just save it and use a .stl extension. From the Surface Display toolbox, you would select the surface you want to save, click on the save icon, and type a filename ending in .stl.
However, I just tried this on my Mac running Monterey (12.6) and it now won’t let me do that. I think this is a recent change with the OS, so I’ll fix that in the next release. It should still work on Linux or Windows.
Give that a try and let me know how it goes. What surface are you trying to print?