Look at the fare rules option relating to each airfare on EXPEDIA. Note the number of permitted TRANSFERS (about halfway down the fine print). For many NW flights it's usually 4/4 (outbound/return).
Finally, at the very bottom it shows city pairs/routings.
You just have to play with the each flight making sure the cheapest fare class is available.
If any one segment does not have the cheapest fare class, it "blows" the pricing for the entire multi-segment trip.

Good Luck.