What I have done in the past is look at the round trip without any stopovers (using the OP by dealguy007 as an example). I would do a calendar search for round trips DEL-IST-X-CVG) and save the dates I found where the fare class & rules matched what I wanted to do. The i would see if I could get the same fares with the travel dates offset by a day or two. At that time you hopefully have all the dates needed to start plugging in the dates into the multi-city search. Obviously an understanding of the fare rules for the specific fare you are trying to use for the stopover is required so your calendar searches return dates that will work within the overall framework.

I don't think there is only one good way to approach this kind of problem. And I don't think you can do this without having an understanding of exactly what the stopover rules are for the specific fare. But what I'm posting here worked out to be the least amount of effort for me. But as always, YMMV.