Checkbox radial button - very new here

Thanks for taking the time to read this post and offer help.
I'm so new I just found out about view code and the term vba today.

Here's my problem. I have an Excel sheet and I'm trying to make columns d4 to d-29 and e-4 to e29 have a checkbox. The user will either check the D or E column.

I read and found 'view code' answers and tried them and so far, nothing. I need a basics course and I need to know how to add the checkbox.
Here is the view code page info. Please forgive me if I have the wrong tags...I'm not sure what that means. Does it mean quote marks?
thank you,
Tracy

_______________________

  Private Sub Worksheet_SelectionChange(ByVal Target As Range)

    If Target.Cells.Count > 1 Then Exit Sub

        If Not Intersect(Target, Range("D4:D29,E4:E29")) Is Nothing Then

            Target.Font.Name = "Marlett"

                If Target = vbNullString Then

                    Target = "a"

                Else

                    Target = vbNullString

                End If

        End If

End Sub

Option Explicit
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Target.Cells.Count > 1 Or Intersect(Target, Range("D4:E29")) Is Nothing Then Exit Sub
    Target.Font.Name = "Marlett"
    Target = IIf(Target.Value = vbNullString, "a", vbNullString)
End Sub