All you need to do is:

1. Pick a time.  Weekend afternoons or evenings work well.
2. Pick a place.  This can be a coffee shop or casual restaurant (e.g., a pizza place or pub) or a classroom or other on-campus location.  Best if it isn’t too noisy.
3. Announce the time and place on LW, a week or so ahead of time, using the "Add new meetup" link near your username.
4. Show up yourself, with a sign that says “Less Wrong Meet-up”.

That’s all -- anything else is optional.  If folks come, just say “Hi, my name’s [whatever your name is]”, and see where the conversation goes.  Most major cities, and many minor ones, have some LW-ers.  And if no one comes, all it cost you was a few hours of reading a book in a restaurant.  You don’t need to have a LW history; many a lurker has enjoyed in-person LW conversation (and the folks who actually show up to meet-ups are often less intimidating than those who post on the main site).

Meet-ups are fun, and the simple act of talking to other LW-ers (in person, where your primate brain can see that they’re real) can help: (a) you become a better rationalist; (b) other attendees become better rationalists; and (c) LW become a stronger community.

Also, if anyone is interested in starting a meet-up but wants to discuss it with someone first, I'd be happy to help.  There is also a good meet-up resources page.

(This was discussed a bit in this comment thread, but it seems worth repeating it somewhere where more people might see it, especially since the idea was new to someone at Saturday's H+ conference who now plans to start a meet-up.)