Make contact with (and make contracts with) a well-recommended sound re-inforcement engineer (and lighting if this is to be a night time event.) Bands are much more willing to play if they know they do not have to haul all their own amps, speakers, etc.

some festivals actually arrange to have radio station broadcast their
event so people can listen even if they cannot attend.

some have their own on-site radio broadcast just with-in the confines of the grounds where the event happens...this is probably more for events that happen in rural or remote places where there is a camp ground for
attendees, so they can hear what is going on on the main stage from a distance.

i may think of more....let us know what you learn along the way!