You should not hold a sit-in until you have worked through the initial stages of educating people and holding one or more protests. A sit-in is a last resort, and should not be done out of the blue. However, if after having worked on an issue for a substantial period, if you are hitting your head against a brick administrative (or corporate or political) wall, then the time might be ripe to strike your target at the heart.
You’ll need a core group of people to start the effort. Organize a series of meetings over the course of which you introduce and discuss the sit-in option. Don’t force reluctant people to commit, but over a couple weeks build up a list of people willing to sit-in. Get them to sign a pledge and repeatedly verify that they are comfortable doing so. Aim for group consensus on the decision to sit-in. Members who do not want to sit-in will hopefully be supportive of those who do, and you need some activists on the outside to organize rallies in your support. Discuss what you will do if security tells you to leave. Scout out the site. It should be strategic (a site of power), ideally near the center of campus (where you can gather supporters), and large enough to accommodate your group of people for a couple days or more.
To avoid security stopping you before you can get in the building, you should keep the discussion of doing a sit-in off all email lists (especially details of date and time). Rely upon word-of-mouth, calling people and group meetings.
