Have a look at the schematic attached, that's where you want to add them for the left and right Channel, i.e. between the two red wires and headphone socket.
I've attached a pic of mine, though a little more complicated as the wiring runs to the XLR socket on the left, then onto the regular TRS socket on the right but the principal is the same, snip the red ones and insert the resistors into the path.