They used a Canadian-based company called Optimal Planning Solutions to draw up the fixtures. Apparently they specialise in sports fixture scheduling and are responsible for the NFL and the American baseball leagues.
Some of the factors they were told to take into account by the SPL were:-
Not having Celtic and Hamilton playing away from home on the opening day.
Not having any club playing 4 out of 5 games either home or away
No long-haul journeys over the festive period, or midweek
A fair balance of home and away midweek games
No clubs playing successive home or away matches several times
No club to play 3 home or 3 away matches against another team