Summarize the following:
Double-click the Excel document that you want to open. It's at the top of the Excel window. A toolbar will appear below the Review tab. It's on the right side of the Review tab's toolbar. Doing so will open a new window. If you want to password-protect all pages in the Excel workbook, click Protect Workbook' instead. Type your preferred password into the "Password" text field. When the "Verify" text box appears, re-type your password into it. Your password entries must match each other. It's at the bottom of the window. This will close the password window. Press ⌘ Command+S to do so. Your PowerPoint presentation will now require a password in order to open. Your document will remain encrypted if you send it to someone, so they'll need to know the password in order to open the document.

Summary:
Open a Microsoft Excel document. Click the Review tab. Click Protect Sheet. Enter a password. Re-enter your password. Click OK. Save your changes.