Open the Excel file
Go to File
Select Info
Click Protect Workbook
Choose Encrypt with Password
Enter a password
Re-enter the password to confirm
Click OK
Save the file
To protect a sheet
Open the worksheet
Go to Review
Click Protect Sheet
Enter a password
Select allowed actions if needed
Click OK
Re-enter the password to confirm
To protect workbook structure
Go to Review
Click Protect Workbook
Check Structure
Enter a password
Click OK
Re-enter the password to confirm