This is pretty easy.
Use Server Admin.app and connect to your server.
With your server selected in the sidebar, click the Settings button at the top, then click Services.
Enable SMB (the Windows file sharing protocol). This sets the machine up to act as a Windows file server.
Save the changes, then click SMB which now appears as a service option below the server name in the sidebar.
From here, click the Share Points button in the tool bar, then click Volumes, then Browse:
Here's where you select the directories on disk that should be shared to network clients. Navigate to your directory and click the Share button on the right-hand end of the toolbar.
Once you've designated the directory to share you can switch to the 'Share Points' tab which lists all current sharepoints. Select your share and you can configure the settings for this share, including access controls and which protocols it should be shared via (options include AFP, SMB, NFS and FTP).
Once you're happy with your settings you can start the SMB server via the button at the bottom of the window.
The whole process seems a little convoluted, and it is, but it's not too hard once you get the hang of it.