It's always been via email from them a few weeks after I made one. The email had a number to be used when booking your "free" flight.
I'm not complaining, but I put the free in quotes beause there were some taxes/fees associated with a ticket that 1-2-3-Free did not pay. Still, a RT ticket for $13.75 (or close to that as best I can remember) is still a pretty good deal in my book...as long as you can find a flight with the required fare class open!