I) Crew enters airport security area through external pathways (e.g. in Duesseldorf you can see them disappear sometimes in some doors near the security checks) 
II) Crew gets checked just like any passenger/person entering security relevant areas
III) Cockpit crew meets to pick up their flight instructions (to-be-flown pathways etc)
IV) Crew (whole) meets in a room to discuss the procedures for the upcoming flight(s) such as who will do which job so everyone knows where the other will be in in-flight problems (etc)
V) they leave either through special bus/transfer terminals to be transported via apron to their respective gate/airplane or take a walk to the according gate (that are the crews you see passing you if you wait infront of the gate on some occassions) 
That’s basically how it works in total simplicity. 