This achievement tasks you with eliminating Lenny's Gang members in the Barber Shop and kidnapping Lenny.
Since this achievement is pretty straightforward and can be completed in multiple ways, I'll just explain how to complete this objective like a silent assassin.
TARGET 1: MASON MCREADY
Head straight down the steps in front of you past the two guards by the door. Pick up the wrench on the metal barrel on the left and detach the wire on the left. Then go to the right and pull the lever.
Go back upstairs and hide behind the short wall near the entrance opposite to the gang. Once you hear Mason complain about the wrong sauce, go back down the stairs.
On the wall of the building the guards are by (right of the lever), you’ll see a ledge. Climb the ledge, turn the corner, and pull yourself up to another ledge. Go to the window and watch Mason die from the electrical current then go through the window and move on to the next target.
TARGET 2: LUKE WHEELEY
After going through the window, go through either door andmake your way to the far door on the same floor. Change into the barber disguise at the bottom of the stairs. Turn right from the mirrors, and go through the door, head towards the kitchen, where you’ll be told that the pizza is for Lenny. Everyone will leave soon giving you an opening to turn on the oven that Luke is near. While you’re doing this, keep an eye on Luke on your radar. Hang out in the storage room to the right of the kitchen and wait for Luke to return. When he does return, stay hidden in the storage room and wait for him to stand next to the oven again.
TARGET 3: PACIFY LENNY
Wait for everyone to settle down by the mirrors, and eventually Lenny will be ready for his haircut. He will sit in his chair, interact and a cutscene will pop.
If you follow this guide correctly you will get "Like Stealing Candy From a Baby" and "Silent Assassin" if you haven't already.
EDIT: On an unrelated note, as of the 21/11/12 I highly advise anyone playing the storyline to regularly BACKUP their saves, as their is a corrupt save bug going around, (Its happened to me twice) this is just an unrelated warning, so the same doesn't happen to you.