With the Litton 72 system on the DC-10, you would agree which waypoints you were initially going to enter and allocate them numbers 1 to 9 sequentially, on the plog. Waypoint 1 would be the co-ordinates of the departure airfield. One pilot would read out the co-ordinates of each waypoint, off the plog, and the other pilot would insert them, monitored by the flight engineer. As each waypoint was inserted, all three INSs would be examined to confirm that they were displaying the correct co-ordinates before proceeding to insert the next one. (In my airline, the procedure was then to circle the appropriate number on the plog to confirm this.) Having inserted all nine waypoints, you then went back to the beginning and checked the True tracks and distances between the waypoints, inserting a tick on the plog next to the appropriate circle to confirm. All very tedious, but necessary.
The departure was always flown in conventional VOR/ADF type nav., with the Flight Guidance System in Heading Select. One very good reason for not putting it into INS, was that in INS mode, the CDIs referenced to True and the heading bugs parked in the six o'clock position. Not a good situation if you were given a radar heading! However, once cleared direct somewhere, you put the FGS into INS, confirmed the number of the waypoint cleared to e.g: "MASIT is Number 8, confirm?" "Check" from the other pilot. You then pressed Track Change 0 (Present Position) to 8, pause, "Check" from the other pilot, Enter. A further careful watch was made to ensure that the aircraft took up a sensible heading.
This action would render waypoints 1 to 7 obsolete. However busy you were, you would have to find time to insert and cross-check at least the new waypoint 1 before overflying waypoint 9 and in time the new 2 through to 7. (You might be busy, as the non-handling pilot, re-writing the tracks and distances on the plog because the oceanic track clearance was not the one planned and filed for.) Always good for a laugh that one...
The whole laborious process was repeated again and again and again as the waypoints became obsolete in turn, all the way to destination. Reversion to conventional nav. and Heading Select was at some point on the STAR, or even earlier, if the INS position had wandered off too much. (This position was the "Triple Mix" position of all three INSs, assuming you remembered to do it.)
The Litton 92, which was available as a retrofit, had a database and made life considerably easier.
(Thanks oxford blue, that'll teach me not to take a lunch break in between starting something and finishing it!)