Thief

Very simply put - pick pocket the guards with pouches on them. You can tell they have a pouch (sometimes more than one) by using focus to see that there would be a darker blue-ish tint around their belt area. Good thing to note is the guards in the city will refresh their pouches as you finish chapters and client jobs (the metal man and the carnie).

If you are thorough and pick pocket everything you see multiple times (take your time in the city, explore) you should have this achievement by Chapter 5.

For the running total of pick pockets, xWaZZySsiKnEsSx says ...
...go to the screen where the first option is "objectives" and select the player progress option, from there you can cycle through all missions with RB/LB and hit the "overall" progress screen where it shows total pickpockets...

ADDITIONAL NOTE: All picket pockets count whether it is in the story or in the city. There is a running total on the 'Objectives' screen for city, story, side, and total.

To answer the question of multiple packs on one guard (sometimes up to three on a person) - does it count for each pack or just the person being pick pocketed? It counts for each looted pack. If a guard has three packs on them, it is three separate pick pockets.

dainja rass adds... After chapter 8 you are given an option to continue exploring the City (pressing A) . It's what I did so I could go looking for collectibles and stuff , and noticed my overall pickpocket counter was still going up .

If you think finding pockets to pick throughout the city when they're relatively sparse in number to begin with is a pain in the rear, You'll like this method I figured out that got me this achievement much faster.

This occurs later in the game so don't push yourself to pick pockets until this if you don't feel it's safe enough at the time. I had gotten through chapter 6 and went to Vittori for his last mission, "Sideshow Attraction". I don't know when exactly you can first access this mission, but you will need to finish his first two before this one is available.
Edit - According to comments below, this mission is available as soon as after completing chapter 5.

When I first started this, I had maybe 40 some total pockets picked. It took me only another four replays to complete this achievement. Add about three more replays if you don't have any at all. That's a little better than an hour depending on how quickly you complete the level.

I noticed almost every guard had multiple pockets to pick in this mission. I ended up with a total of 16 pockets picked and the mission only took a little over 10 mins to complete. I can confirm that replaying missions/jobs, while it does reset the count for that job until completed again, it adds to the overall stat.

The accompanying video shows you exactly how I go about getting through this mission undetected, finding all the loot(only 18), and completing another challenge as well.

You'll see in the video at the end my total stat is well above 100 now thanks to this.

Video will be available shortly. Youtube is processing the final edit.

For players who explore extensively, this achievement will be earned in the course of play. However, for people who play in a streamlined fashion, the game can be finished well short of the 100 picked pockets for this achievement. The achievement can be earned fairly early by employing the Pickpocket Loop shown in the above video. It is done in the vicinity of Garret's home, the Clock Tower.

The video shows game play using the highest difficulty level--a Master custom difficulty greater than 700 points--to demonstrate that the pickpocket loop will work on any difficulty setting.

Note: I made the video playing the PS4 version. However, it is exactly the same on the XBox One. Pickpocketing is performed by holding down the interact button until the purse is collected. A prompt, "Hold " will appear when Garrett is close enough to a purse carrying target.

The following map depicts the path taken by Garrett in the above video.



The pickpocket loop starts from the hideout in the Clock Tower and runs a clockwise loop back to the Clock Tower. Along the way, Garrett can pickpocket four guards and one civilian. Each pocket picked will net him 12G. The loop takes a little over three minutes.

Referring to the map above, the section on the left depicts the beginning of the loop when Garrett is above street level. The section on the right depicts the second part of the loop after Garrett descends to street level. As is to be expected, all five picked pockets occur at street level.

As shown on the left hand side of the map, Garret leaves the Clock Tower and heads South. He takes the first left to head East. Right after this left turn, he has to make an adjustment to his left front to stay on the walkway. At the end of the walkway, he has to crouch down in order to make a right hand turn on another walkway running South. At the very end of this walkway, Garrett vaults the railing landing on a sloping roof and slides to street level as indicated by the red dot.

Referring to the right hand side of the map, Garrett is now at street level as indicated by the red dot. He crosses the street moving West into an enclave. At the West side of the enclave, he climbs on a crate and then over the railing. He makes a turn moving left, crouches near a crate, and sees two guards moving East to West on the street just to his South. The guards will not see him from that distance. He crouched, not for stealth, but to prepare himself for the first pickpocket. (If you do not see the guards, you’re slightly behind schedule but you have quite a few seconds of slack here so, unless you were very slow, there should be nothing of concern.)

There will be a civilian standing against the wall Northwest of the crate Garrett is crouching behind. He will stand straight and start to walk. He will look straight at Garrett. Like most civilian activity, this is of no concern. The civilian is not even curious much less suspicious. Move behind him and pickpocket him. He is much more insensitive than the guards. Unless Garrett really disturbs him, he will remain oblivious of what is going on. Garrett must, however, pickpocket him before he moves more than a few steps away from his crouched position. There is another civilian (with no purse) further North. He will become suspicious if he sees what Garrett is doing.

Reminder: Crouch while pickpocketing. You are very susceptible to detection when not crouching.

After you pickpocket the civilian, Garrett goes to the South end of the alley and crouches against the West wall. (Note: In the beginning, you may want to make a manual save you can revert to here in case of error). Peeking, Garrett sees the two guards standing West of him. After a while, they begin to move towards him. He moves back in his crouching position and waits concealed by the West wall.

Don’t be too hasty. If you are playing on Master custom difficulty, the guards are quite alert. If you approach too diagonally, the guards will sense this and detect you. You will want them to be a couple of steps East of you and try to approach them straight from behind. You can pickpocket them in any sequence. I usually find it easier to pickpocket the one on the left first.

The goal is to pickpocket them quickly and then go to the North end of the street Garrett just came from. He will then be just in time to see two guards just below. Garrett drops down behind the two guards. Make sure you are crouched because they will detect you if you are not. You have a lot of time to pickpocket them. You can do one or both quickly while they’re both still climbing the stairs or you can do the second guard or both while they are walking their long route North towards the Clock Tower.

Before the two guards stop, duck into the enclave to your left. It is on the West end of a street going East into the plaza. Wait for them to stop, turn around, and pass you going South. Then, go to the point where they stopped. There’s a climbing place against the wall. Climb up, scoot to your right, climb over the railing, and turn left to go back into the Clock Tower hideout.

Going into the hideout creates a checkpoint save. So does coming out of it. Whenever Garrett comes out of the hideout, everything is reset--purses are full and people's locations and movements start from scratch. So you are in perfect position to keep repeating the loop until you meet your pickpocket goal.

There are two ways to this acheivement

Stealth or Force

Stealth Method

Use focus to see if the target got a pouch (its around their waist). sneak close to them and hold X to pick pocket them.

Force Method

Use your blackjack and force them into kneeling, usally you would just finish them off by holding X but instead be fast and go around their back and hold X to pick their pockets before they stand up again, when you have taken all their loot then finish them off.

This can be done on guards in the city and on any chapter or job. Even after you complete the game you can still go for the acheivment.