Well, it's just like making a regular reservation. The most user-friendly interfaces for booking and cancelling are offered by Delta Airlines. You can therefore use the following underlying techniques for this:
Visit Delta Airlines' official website.
After that, for booked flights after that.
the
Delta Multi City Flights travel category next.
Moreover, indicate how many passengers you are making the reservation for.
Enter the flight one departure and destination cities, as well as the trip dates.
You must now decide when your first flight will depart.
The next step is to select the second flight's departure and destination airports as well as the connecting airport.
Source:
https://manageairlinesbooking.com/blog/how-to-book-multi-city-flights-on-delta-airlines/