It is more or less unheard of for a Man Utd manager to resign - traditionally they've been sacked after a run of bad results.
But there are still rules. Firstly the call goes out to the Cardinals - Cardinal Glazer, and Cardinal Glazer. Blessed by the oldest non-voting Cardinal, Charlton, they enter into conclave to decide which dour Scotsman to give the job to. Obviously there's no rule to say it must be a Scotsman, but all the non-Scottish holders of the post have been rubbish.
At the end of the first sitting of Conclave, it is decided they need another nine minutes to make their minds up. At this point, the declaration will be made, "habemus Fergie-time."
The Glazers will then go back into Conclave, to make their minds up.
After the decision is made, white smoke will be seen floating above Old Trafford. This is caused by the Glazers burning tenners. Because they can.
After the appointment of a new Man Utd manager, the previous incumbent is expected to slink off to work for Sky Sports, until they are sacked from there too. But, since Sky have already signed a non-English-speaking panellist for next season, Fergie is expected to knock about the place, pledging obedience to his successor, and continuing his life-long search for a watch that doesn't keep stopping.