How to set permissions for sub folders and files

Hi 

I have a question about how to set the permission for subfolder? I don't find the settings of the permission of subfolder, the authentication type of my NAS is Active Directory.

for example

READYNAS
|
|
---- Agency (Share)
       |
       |----- Designers (Folder)
       |        |
       |        |---- Graphics (Subfolder)
       |        |
       |        |---- Web (Subfolder)
       | 
       |----- Developers (Folder)

I would like to be able to control permissions based on folders, not just based on shares. 

If I have a group of graphic designers, I want to be able to give them access to the Agency share, the Designers folder, and the Graphics subfolder. 

If they try to access the Web folder, they will be denied access. If they try to access the Developers folder, they will also be denied access.