To password-protect a spreadsheet, execute the following actions.
1. Access the spreadsheet requiring protection.
2. Navigate to the File tab and select Info.
3. Opt for Protect Workbook or Protect Sheet to secure the entire file or an individual sheet, respectively.
4. Select Encrypt with Password from the available choices.
5. In the ensuing dialog box, input your chosen password and confirm by clicking OK.
6. Re-enter the password for verification and click OK again.
7. Save the spreadsheet to activate the password protection.
Crucially, retain your password, as its loss will result in permanent inaccessibility to the file.