As far as I know, all you have to do is pay the settlement. If they have an online system, it will ask you for the case reference (or similar) so that they will know what the payment is for. If they want a cheque posted to them, then
include a covering note giving the case reference write the case reference (or similar) on the back of the cheque just in case.
However you pay them, keep a note of when and how you did it, just in case something goes wrong: that way if there's any argument you'll be able to show that you have paid after all.
(Edited to include
@Titfield 's sensible point from post #74 below.)