Sometimes I work as a sideman or solo guitarist. Sometimes I am the band leader. When I lead a band, this is how I approach it.
1. Work with the event coordinator/venue manager to get the specs of the gig (date, time, size of band, number of sets, sound system, style of music, etc.).
2. Round up the right musicians for the gig, and secure a sound system/sound tech (if not provided by the venue).
3. List all the songs the band will be doing
4. Create charts for the rhythm section, lyric sheets for the singers/BGVs, and horn charts (if using horns).
5. Get the charts and MP3s of the tunes to the band as far in advance as possible.
6. Depending on the gig and who is in the band, sometimes we'll rehearse - other times we won't (if its players that have gigged together previously). Rehearsals tend to focus on beginnings & endings of songs, any parts that merit some attention, and transitions between songs (you want to minimize dead time between songs).
7. Send out the set list - with transition notes - to the band.
8 Show up, play the gig, go home.
Good communication is key. It's important to get all the facts in writing from whoever is hiring you to avoid incorrect assumptions. This can be anything from an e-mail to a signed contract. Once you've been burned on things like, "Yes, the band will be under a tent" only to find yourself in the blazing sun, or "Yes, we have an adequate sound system" only to show up and find they have no clue what "adequate" means, you will quickly learn to put the facts in writing.
Herding a bunch of musicians to the starting gate can be a challenge as they vary greatly in dependability and professionalism. If you are hired as a sideman, make sure you know the material, show up on time with a good attitude, and be willing to lend a hand (e.g. help the drummer haul his set in, etc.), don't jam during sound check/set up (this will not impress anyone), etc., etc., etc...
Its a lot of work, but a lot of fun. I find the more work you do on the front end as band leader (making charts, etc.) actually reduces the effort required to have the band crank out some great music.
Here is a video showing a few clips from a recent gig on which I was the band leader. The quality isn't that great as it was shot with an iPhone - so just pretend it sounded better :-)
Below is the set list with transition notes. The transitions are just as important as the songs. It is important to keep a flow to the set and not have any dead time between songs.