the instruction say "Protect the worksheet so that only cells with data can be selected"
I understand I must protect the worksheet but how do I make it so ONLY CELLS WITH DATA CAN BE SELECTED?
first, you need to format all cells that should be selectable. In the formatting dialog, go to the Protection tab and untick the option for Locked.
Next, go Review > Protect sheet. In the dialog that appears, there are options for which selections to allow. One is to allow selecting only unlocked cells.
Does that help?