In general, when I book award tickets on other carriers using DL Sky Miles, I find it actually easier to call and agent and book directly over the phone. Other way to do it is to search exclusively the DL operated flight segments for the lowest mileage availability. Then go to expertflyer.com and search for the low AF award fare class on the purely AF routes. Then try to have a phone agent link all the segments on the days you wish to try. I'm less experienced with the DL award web site since the NW merger, but I'd be surprised if the site came through with low award levels on that many segments. If you are not medallion, it may still be worth capitulating and paying the agent ticketing fee to have that itinerary done over the phone if you can net the lowest mileage levels.