1. activate system
2. Move ships (note: no ships need to be moved to cause PDS fire, just activation of the system. Of course, if a player has no fleet in the target system than there are no casualties obviously)
 
 
3. The rules say "enemy PDS" which is translated to other players' PDS units. Any PDS in range can fire. Emphasis on anybody's not just the player in the system already or the player moving into the system. In range means PDS in the system as well as PDS in adjacent systems where the PDS controller has deep space cannon tech. Enemy (i.e. the non-active player or players) PDS fires first then the player who activated the system can fire his PDS.
 
 
FAQ says:
-You can fire PDS from a system that was previously activated
-You need not move a fleet into a system to use PDS, as a matter of fact you can activate the system for the sole purpose of PDS fire if you desire.
-If the enemy activates a system in range of your PDS you can fire at fleet units owned by that activating player ( so third parties could fire if in range but only at units of activating player.)
-PDS can fire multiple times (once when enemy activates and on a subsequent round where you activate the system, or if you use main rule Warfare SC to remove counter or an AC to remove counter and then activate the system again.
-You can still use PDS to fire on an enemy in range who activates a system if they have used the diplomacy primary ability this turn.  You however cannot activate an enemy system containing units as per the diplomacy SC.
 
Note that certain ACs and SCs may cause ships to move and/or activate the system you need to pay attention to these effects since they may not go through the normal sequence of a tactical action therefore there may not be a PDS fire phase.