A: You add the contact to the segment that's connected to the flow. See Q: Can I add contacts to a segment manually?
It can take up to an hour before a contact added to the segment shows up in the flow. Emails in the flow are only sent during the send times set for the flow, and not on weekends if Don't send on weekends is selected.
A contact that has been in the flow before doesn't start it again when added back to the segment, unless the flow is set to Loop flow. See Q: Can a contact go through the same flow more than one time? and Q: Can I make a flow start over again?