Click the Options button. That leads you to this window:
(Naturally it is completely different from what you see when you click Options in Microsoft Word. If you want to know how to password-protect a Microsoft Word document, click here.)
If you set a password for opening the document you’ve really done all you need. You can probably come up with a situation where you’d want one password for opening the document and another one for editing but I think that situation would be unusual. Anyhow, enter a password for opening the document, click OK, and you’ll see this box:
Enter the password (this is to double-check that you really know what the password is) and you’re all set. From then on, double-clicking the document results in a box like this:
Enter the password and you’re in. Get it wrong and you’re kept out. Easy as pie. The password is stored in the document, so if you email the document to someone he’ll need to enter the password to open it up.
That’s all there is to it. But remember the password, because if you forget it, you’re sunk. I don’t know anyone who can hack into a locked Excel document– I don’t think it is possible. Maybe you should make a spreadsheet of passwords. Password-protect that one too.