Pre-production
1. Pre-production begins when a script is approved. A director is chosen to plan the episode’s final look.
Pre-production tasks include storyboarding, construction of sets, props, and costumes, casting guest stars, budgeting, acquiring resources like lighting, special effects, stunts, etc. Once the show is planned, it must then be scheduled; scenes are often filmed out of sequence, guest actors or even regulars may only be available at certain times. Sometimes the principal photography of different episodes must be done at the same time, complicating the schedule (a guest star might shoot scenes from two episodes on the same afternoon). Complex scenes are translated from storyboard to animatics to further clarify the action. Scripts are adjusted to meet altering requirements.
Source: Wikipedia