Research the musical and spend time planning how you want to stage the production. In researching the musical see how other directors have staged it in the past and, if a period piece, understand what was going on in the world at the time the musical is supposed to take place.
Create a rehearsal schedule that will run six to eight weeks. Rehearsal schedules need to include days off, fittings and tech rehearsals.
Distribute scripts, musical numbers and schedules to cast a week before rehearsals are to begin.
Schedule your costume fittings a week or two from opening night. It would be best if these fittings are scheduled outside of regular rehearsal times but if that's not possible just have a rehearsal day set aside for costume fittings.
Schedule the tech rehearsal the weekend before opening night. Tech rehearsal can be long and tedious, so splitting it over a Friday and Saturday may be best for everyone.
Introduce the cast and crew, go over the schedule, discuss expectations for rehearsals and the production in the first rehearsal.
Spend the first three weeks of rehearsal blocking the script and musical numbers.
Give the cast notes after each rehearsal. The notes should cover things they need to work on for the next night's rehearsal.
Take actors off book after three weeks. If actors need prompting, the stage manger or assistant director can prompt them as needed but actors need to know their lines by this point.
Spend dress rehearsal making costume adjustments, fully integrating props into the production and making any changes in blocking or to the set changes that may be needed. Dress rehearsal is about getting the timing of your production just right and it's your time to fix the musical before it goes in front of an audience.
Give the cast the day off before opening night. This may seem a bit unconventional but it gives the cast and crew a chance to relax and rejuvenate from all the hard work they've been doing in rehearsal.