Warning / Error alert

Hi. You would simply add the additional conditions.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If ActiveCell.Column = 12 Then MsgBox "Check you have used the correct column"
    If ActiveCell.Column = 4 Then MsgBox "Check you have used the correct column"
    If ActiveCell.Column = 5 Then MsgBox "Check you have used the correct column"
End Sub

Or you can shorten it up a little bit:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If ActiveCell.Column = 12 Or ActiveCell.Column = 4 _
    Or ActiveCell.Column = 5 Then MsgBox "Check you have used the correct column"
End Sub

Thanks for all your help. You have all been great!