Refund credit from non refundable fare

Good day all, I have been searching and can not quite seem to find an exact answer so here goes. I had to cancel a flight to LHR for reasons other than what would get me a refund waiver (K fare). So I got the email saying that while it is non refundable you will have the credit to use for future travel. There is no ecert code or anything in the email. How exactly do I tap into that credit, and does it all have to be used on one flight or can it be spread out for two different reservations (have a few domestic flights coming up).

First time in this situation, so just making sure I do not screw this up.