The process during WW2 involved putting everyone in a room until they had formed themselves into crews. On my first OCU (242 OCU) we were also expected to make up our own crews but over the first week of the course as I recall rather than in an afternoon. On subsequent courses and when I was on the OCU staff myself crews were selected to allow for experience, possible CRM issues etc, sometimes with a planned change mid-course so that any issues that did crop up could be corrected without any hint of embarrassment to the individuals concerned.
On my first OCU the Nav was transferred out of our crew and replaced as it was perceived that he was a ‘weak link’ affecting our overall performance which did result in a fair amount of negativity for all of us.
Once on the squadron crews were made up for each trip as described above, with planning done for the month ahead although that was always subject to a considerable amount of change as the crewing ratio was much lower than for an airline so sickness etc had a major knock-on effect. The shortest notice I had for a route was 20 minutes for a call out when they couldn’t contact the standby co-pilot so I was phoned up as I lived on base.