It's a process but not hard.
I open 2 windows
I make sure the Ding is available and book right up until you pay.
I then cancel my original flight and get the TTV (same as confirm number).
Then switch back over to Ding and put that as the payment. The rest will remain as the TTV for future use( 1 year from booking not travel date).
You also know it's better to book just one-ways so that it makes it easier to change if a Ding only comes availible for one of your flights.