Determine the location where the flash mob will appear. Some mobs are formed more formally than others. The assistance of a venue or of musical performers already staging a gig can come in handy, but many mobs are done as a total surprise to everyone at the location.
Secure a rehearsal space. If you plan to do a flash mob with hundreds of people involved, you can probably rent a public rehearsal space for a few hours. If you prefer, meet at a park to choreograph the plan and discuss all pertinent details.
Schedule the date and time for the event. If you're going to do more than one in a day, be sure you allow time for everyone to commute between locations.
Get the word out. Post anonymously on Craigslist, Facebook, MySpace, Twitter and other social sites to let people know you're forming a flash mob. If you're doing a formal mob, write up an "info sheet" with your contact info and details. Send this to responders. Provide the date and time, but not the location of the event. Advertise wherever you can, including on bulletin boards and signposts.
Rehearse. Either early on the day of the event or a day or two beforehand, get everyone together. Welcome them, then give an overview of the plan. Turn on practice music or hand out copies of lyrics if people don't all know the words. If they're only going to dance, omit the handouts. Lead them, or have a separate choreographer lead them, through the steps of the dance the flash mob will perform.
Discuss the cues for when the first people will begin and when others will join in. You'll only need a couple to get things rolling.
Go separately to your location at least 20 minutes before the mob will begin. Set the cues in motion, and let the fun begin. Disperse immediately into the crowd when the routine is complete.