To make them Attack: There is no scripted method to make them attack you as they randomly appear in the map. But they normally spawn where you see patrolling guards in large number, like 3 or 4 patrolling guard groups. Just walk around in the streets near them & you'll eventually find one.
Really? I remember being grabbed by a stalker from behind a while back, it was the first time so I was like: HOLY ~censored~ WTH?!
And all I did was press LMB in... 'shock'. And Ezio stuck his hidden blade right into his face. Maybe I got lucky?
as to force attackt here are "mostly" random, more attack if higher wanted i find (all red) and i belive mission 5-2 forces one in your path, not sure exact mission number though but one has a scripted stalker event as part of story to help farm on