A lot of people have been hitting this blog with the above title as a search term.
For me, the tough part is doing the breakdown. I have several posts on that already, but I will recap briefly:
Use French Scenes if you've got them. A costume chart is a good alternative if not. Failing that, find out from the Music Director who she is asking to sing certain numbers; this way you get the key people in the ensemble. You see; figuring out when principles sing is easy. Figuring out which ensemble members need to be on mic at any one moment is more difficult!
And you will almost never have as many mics as you have cast members. My preference is the fewest microphone switches the better. When you change actors, there is a chance for the microphone to be damaged, as well as a chance the change will be missed or forgotten -- meaning instead of the chorus member you wanted, you get dressing room chatter. Not fun. Also, multiple assignments means you can't trim the mic as tightly to each voice (well, depending on what options your sound board gives you.)*
A French Scene, if you don't know of it, is a breakdown with the scene number along the top and the actor name along the left side, with the character they are playing written in for each scene in which they appear. So for principals this may be the same name across the sheet, or perhaps "Mr Darling/Captain Hook/Mr. Darling." For ensemble it will be more like "Maid, Pirate, Indian, Pirate, Jenny, Mermaid, Pirate..."
You can actually write over this with blocks for each microphone. Otherwise you make a flipped version of this; a chart with the scene number on top, the microphone number on the left side, and the person assigned to it by actor name in the blocks. Spreadsheet programs are great for this, and I've never been disappointed by the freeware OpenOffice.
For handing out to actors and mic wranglers I've found this is more detailed than is needed; just list the mic number and the actor(s) assigned to it, with a notation as to when the switch needs to happen, aka;
#9 Brad G.
#10 Suzy B/Karen M (switch during actI scene 3)
#11 Marsha B/Marsha M/Marsha B (switch at intermission, get back to Marsha M before ActII scn3)
There's one additional wrinkle. What if you are double cast? In almost all cases it makes most sense to keep the mics with the character, not with the actor. So Mic #2 will always go with the character SARAH and will be marked that way on your sound board, even if two different actresses are swapping out. This seems to work most smoothly even if you have a single cast swapping roles on alternate days; the downside is of course that you have to tune the mics to the appropriate voice each performance.
Of course, double casts are not always identically arrayed. One cast may include more ensemble members, one cast may split a role that was handled by a single person in the other. So these are the times you really need to keep individual paperwork for each performance.
The thing to remember is that things will go wrong. A mic will die. Someone will get the wrong mic. Someone will take sick and there will be a last-minute substitution of understudies and merging of ensemble parts to cover the gaps. So the guiding principle in any scheme of charting and mic assignment should be that the person mixing the show will always know what role will be under each finger when the next song comes up. So that's why I default to character labels, even when it would seem neater to have the same actor keeping the same microphone even when they are singing a different role that night.
* A word on this. Voices change from night to night. And I'm a tweaker anyhow. So although it is easy to memorize trim and EQ with every scene on a digital sound board, it works better to adjust to that actor's performance on that night through the first couple of songs, then keep those settings through the show. And retain them until the next one, assuming you liked them and think they will make a good starting point for the next performance.
So what's the alternative? On something like the Yamaha LS9, you can assign a single input to multiple input channels. So Mic #1 can also show up on the board as Mic#14 with different trim, EQ, and digital effects. I've found this extremely helpful for those times when you want special processing for a scene or a moment, but there just isn't the board space to assign all the mic swaps to individual channels (not and see the live band which I'm also trying to mix...)