Grey out checkboxes until name is selected - Userform

I have a user form that staff must select their name from a drop down menu.
Once they have selected their name they can then check boxes that relate to shifts they wish to work.
How can I have all the check boxes greyed out until a name is selected from the drop down list.
Sometimes staff are selecting their Shifts and forgetting to select their names. I would just like to make it simpler for everyone.
My check boxes are numbers CB01 through to CB25.
How would that code be written and where would I place the code.
Thank you for any assistance/guidance that you may be able to offer.