If you wanted low-level technical details, this is how I do it:

I have a phone number to a cafe, where I already did some meetups, so I call them to reserve a table for me on Monday from 18:00. They have no problem with it, even if I say "I am not sure how many people will come, at most 10 but maybe much less", because on Mondays they are half-empty anyway.

When I did this the first time, I used google and recommendations from my friends about nice places in the city. Then I visited personally (to see the place, and to be able to negotiate in person) and asked whether it would be okay to have a table reserved there. -- I specifically emphasised that most participants will be students who don't have much money and just want to talk, so they shouldn't expect a big revenue from this; and I asked whether that would be acceptable. (It's better to have a feedback in advance than a misunderstanding later.) -- Then I picked a Monday cca 2 weeks in future.

I post an announcement on LW, and also to the local LW mailing list. Sometimes I send personal e-mails to people I think could be interested. (Mostly they don't come, but sending the e-mail is so cheap and sometimes they come, that it's worth doing. Some people don't come the first few times, and then when I already lost hope, they appear.) -- At the beginning I also used to post an invitation on my Facebook page.

A day or two before the meetup, I send e-mail reminders. The e-mails contain my phone number, in case someone would have last-minute problem finding us.

I go to the meetup. -- At the beginning, I was careful to be the first one there, and to somehow attract other people's attention (e.g. have a paper saying "LessWrong" on the table). These days we already know each other, and we meet in a cafe that has only three tables, so this is not necessary.