1) Install samba
> emerge net-fs/samba
2)Edit cfg file - /etc/samba/smb.cfg
Just add new share at the end of config file by example.
Nothing to do additional.
3)Add guest user to the system
> useradd -m guest
4)Set password for guest in system
> passwd guest
5)Set password for guest in samba users db
> smbpasswd guest
6)Create share directory in guest home directory (under guest account! must have access!)
> mkdir /home/guest/share
7)Set executable permision to share directory
> chmod +x /home/guest/share
If you need another location of directory, play with access permision and user accounts.