Jubair is stationed at a particular site in the southeast of Damascus. If you've done a thorough investigation, your memories should uncover a series of maps crossing out stations where he wouldn't be. Merging those maps would give you the exact one.
The building Jubair's stationed in is open-aired so get to it via rooftop and climb down the conveniently-placed ladders. Careful to keep his back to you as you move or he might see you. Climb all the way down, hidden blade stab him, and you've won! Just get away from those guards and back to the Assassin's Bureau... ;) (This is also the easiest way for people going for the Disciple of the Creed achievement.)