What two things must happen to cause an silicon controlled rectifier to fire?

You must have a voltage on the gate that exceeds the turn-on threshold of the SCR. Typically around 0.65v. Then you must maintain a minimum holding current from Anode to Cathode. Once turned on, the SCR will remain self latched in the on state until the current through the Anode and Cathode falls below the minimum self holding current.