Open the Excel workbook
Click File
Click Info
Click Protect Workbook
Select Encrypt with Password
Enter a password
Click OK
Re-enter the password to confirm
Click OK
Save the workbook
Close and reopen the workbook to verify the password prompt
Open the Excel workbook
Click File
Click Info
Click Protect Workbook
Select Encrypt with Password
Enter a password
Click OK
Re-enter the password to confirm
Click OK
Save the workbook
Close and reopen the workbook to verify the password prompt