First, post your audition schedule. Depending on the type of audition, you can hire casting people to advertise your auditions, you can ask select people to audition or you can post a general notice, either with posters or online.
Film each new audition if you're casting a movie. You'll need lighting, a camera, a camera operator and a casting person to run the audition. You may also want to have a "reader," an actor who reads opposite the auditioning person. For film, use a cold reading or give the actor sides to prepare ahead of time.
Allow actors to prepare monologues for new auditions for a play. You'll need an audition space and a stage manager to run the audition process.
Arrange to have both singing and dancing rooms available for a musical audition. In addition to the director, you also need a musical director, a choreographer, and a stage manager to run the auditions. For a musical, have the actors prepare a song, and learn the dance onsite.