ringo, as below:
On the NAS open the "Shared Folders" app and click create, chose a name and assign who can access it (this will look at the users created on the NAS, the default being "Admin" and "guest". you can choose read, read/write and no access)
this doesnt depend on the nas but the client, windows will remember credentials provided you are nagivating to the same place. after authenticating once you will only need to type \\server\share and it will just open
Again depends on the client, mapped drives within windows will "reconnect at login" unless you tell them not too. Mapping the drives ive found more useful as this assigns them a drive letter within windows, therefore windows backup can use them as if they were a local disk