Direct Message (DM) the tag and time to everyone you trust. Include “pass on in DM” or “share through DM only” in the message so everyone will get the idea.
Only invite people who will be friendly to the idea. Avoid anyone who might try to crash the party or expose it too soon. Keep an eye on the tag in Twitter search to see if anyone starts using your tag early, then DM them to kindly cease until the right time.
You can only send DMs to followers, up to 250 per day. Make sure you are following everyone you invite so they can DM back. See who responds to your DMs and add anyone who seems excited to participate to a list for future Twitterbombs.
We used the above Direct Message to set a Twitterbomb for 8PM last night, and #GunsOverPeople was trending Worldwide by 8:11PM.