On the File menu, click Save As, then click on the down arrow next to
Tools(On the save as menu).
On the Tools menu, click General Options.
Do either or both of the following:
If you want users to enter a password before they can view the workbook,
type a password in the Password to open box, and then click OK.
If you want users to enter a password before they can save changes to the
workbook, type a password in the Password to modify box, and then click OK.
(You can also select to open the file as read only, and to always make a
backup)
When prompted, retype your passwords to confirm them.
Click Save.
If prompted, click Yes to replace the existing workbook.