Help with making a drop down list in many cells that allows me to select multiple options

I'm a translator trying to track and classify my work project. I want to create a column in which each cell has a list of subject categories (Legal, Business, Education etc) and I want it to be possible to be able to pick multiple subject categories. There are a number of "how to" guides online that give code but I have tried entering it where they tell me to and it's not having the desired effect. I have made the drop down list in one cell already. I just need to apply it to all cells in the same column (to infinity as I will keep adding jobs over the course of my career) and I need it to let me select multiple subject categories (I want to class a supplier contract under both business and legal for example.

I tried using this code ,changing only $C$2 to the box in my spreadsheet already containing the list ($K$2) This didn't work and I'm not sure why (although I have no coding background - I'm just trying to teach myself the basics: Thanks!

Private Sub Worksheet_Change(ByVal Target As Range)

'Code by Sumit Bansal from www.trumpexcel.com
' To Select Multiple Items from a Drop Down List in Excel

Dim Oldvalue As String
Dim Newvalue As String

On Error GoTo Exitsub
If Target.Address = "$C$2" Then
    If Target.SpecialCells(xlCellTypeAllValidation) Is Nothing Then
    GoTo Exitsub
    Else: If Target.Value = "" Then GoTo Exitsub Else
        Application.EnableEvents = False
        Newvalue = Target.Value
        Application.Undo
        Oldvalue = Target.Value
        If Oldvalue = "" Then
            Target.Value = Newvalue
        Else
            Target.Value = Oldvalue & ", " & Newvalue
        End If
    End If
End If
Application.EnableEvents = True

Exitsub:
Application.EnableEvents = True

End Sub

I am not sure of what you are trying to do.. It is always better to upload sample workbook to illustrate your issue well
As for dealing with whole column you could replace this line

If Target.Address = "$C$2" Then

With this line

If Not Intersect(Target, Columns(3)) Is Nothing Then

This will allow you to deal with any cell in column C not only C2
Hope this will help you