Follow your map marker to the southwest district. Blend in with the group of scholars. Once you are inside, just stand with the crowd and watch.
Once the funeral is over, prepare for battle. You're going to have to kill a lot of Robert's guard just to reach him. If you are proficient with the Hidden Blade counter kill, now is an excellent time to use it.
Once you've dispatched enough of Robert's men, dispose of Robert in any manner you see fit.
Once you've viewed the rather interesting cutscene, just make your way back to the Assassin's Bureau for the achievement.