This, and much more, is in out setup manual. You better read it.
Create a folder to be the shared folder in a convenient location.
Start SheepShaver and in SheepShaver preferences, Volumes tab, Unix Root: Add the path to your shared folder using the "Browse…" button to find and "Open" the folder you created to be the shared folder. (In the example below I used a folder named "Shared" on my desktop.)
Save the change, quit SheepShaver and launch SheepShaver again. The shared folder will appear as a volume Unix on the OS9 desktop.
See also the instructions in the manual about the use of the shared folder:https://www.emaculation.com/doku.php/sh ... _unix_disk