This is already explained in Tips n tricks 3rd page.
*www.thinkdigit.com/forum/viewtopic.php?t=15125&start=40
In short,
How to password-protect GRUB(*linuxgazette.net/107/tomar.html)
* Open a shell prompt and log in as root
* Type /sbin/grub-md5-crypt and press enter
* Enter the password you chose for GRUB when prompted. This will return an MD5 hash of your password
* Open /boot/grub/grub.conf in your favorite text editor
* Add password --md5 <password-hash> below the timeout in the main section (Replace <password-hash> with the hash you got in the previous step)
* Save and exit
* The next time you reboot, the GRUB menu will not let you access the editor or command interface without first pressing [p] followed by the GRUB password.
for more try in CLI "info GRUB" or "man grub"