I don't actually know what the real operating procedures are for speedbrake use during flight, but here is where I use it:
Descending at cruise speeds
When you command the autopilot to fly at a lower altitude, it will typically choose a descent rate that adds about 10kts to your target airspeed. Before initiating the descent, set the target speed down 10kts and then use a little speed brake to bleed off that 10kts before retracting the speed brake and descending. If you aren't in a rush, then don't use the speed brake, maintain level flight until the speed has dropped off. If you are controlling the descent manually you control your airspeed with pitch changes and can fly a lot smoother.
Descending to Approach/VFR airspace
ATC will give you altitude changes that require you to descend at a much faster rate (giving 20-30kts extra airspeed) and you will need to comply quickly. You may not have time for the engines to reduce thrust and even at idle you may still increase airspeed in the descent. Initiate the descent as before but this time don't retract the speed brake, continually monitor airspeed and change the brake setting if necessary. When leveling out, retract the speed brake. In VFR airspace you will need to fly below 250kts, and even slower if you are maneuvering for approach, you will need to slow down during your descent. In this case, modulate the speed brake during the descent to slow down gradually as you descend.
Approach and Landing
First check that the speed brake is retracted. Don't apply the speed brake in level flight at approach speeds. Arm the speed brake to deploy on touchdown ( shift + / ). Visually check that it is armed (One of the displays may announce it and the lever will move to 25%). On final, check that it is still armed. On landing check that the speed brake deploys and retracts automatically, intervene if necessary.
Emergency Situations
The only emergency situation I can think of is if you are overspeeding at very high altitude. Normally an overspeed can be corrected with a reduce in thrust and nose high pitch. However, jet engines react slowly and at high altitude pitching up is at a greater risk to stall. I'd use the speed brake in such a case.
Use of the speed brake should be a proactive affair. When you have planned to use it, you have far better control and understanding of the craft. If you have to re-actively use it, then you can expect things to not go so smoothly.