You want to put on a show. None of it is complicated, but there is a lot of it, so here is roughly the order it goes in.
Start with a date. For a venue with a real calendar, give yourself at least eight weeks. For a living room or a DIY space, four is fine. Stay away from the weekends of the big festivals and whatever football final is going to empty the room. Then find the place. A café, a basement, a record shop, the back of a brewery, a friend's living room, they all work. Email the booker, or the owner if there is no booker, and tell them plainly what kind of music it is, how loud it gets, how many people you expect and what the ticket will cost. Do not oversell it, because they have heard the oversell before.
Three bands is the sweet spot for an evening of about ninety minutes. Open with the band that brings the smallest crowd and finish with the strongest local draw, and confirm everyone in writing so there is no confusion two weeks out. Sort the sound out early. Ask the venue what is already there, and if the answer is nothing, you need a small PA, a couple of microphones, a few inputs and, more than any of that, somebody who has run sound before and whom you are going to pay. Good sound is most of the difference between a night people remember and a night they forget.
Put one person on the door with a cash box, a clear way of counting and a proper total at the end. Nobody plays for free and nobody walks off with the takings. Decide the split before the night and write it down, whatever it is, an even share of the door after costs, a small guarantee plus a percentage above break even, or the headliner taking the larger cut. Tell the bands the formula in advance and pay them in cash once the night is over.
The promotion can be simple. A flyer a couple of weeks out, a poster in a few of the right places, one post online with everyone tagged, an event page people can actually find. That is genuinely enough, and you can stop reading articles about marketing now. On the night, open the doors at the time you said, soundcheck before that, and keep the bands to their slots so the last one is off stage in time for you to settle up before the venue wants to close.
Afterwards, pay everyone, take down your posters, thank the venue, and write down what worked and what did not while you still remember. Then do another one. That is the whole trick. It is not about knowing more than anyone else. It is about doing it once, badly, and then doing it again a little better.
Love,
Marten