You unlock the achievement by pickpocketing 20 times
AND without getting caught. If you are caught, the pickpocketing attempt will not count towards the achievement.
Pickpocketing is performed as follows. Press

to crouch, approach a character (enemy or non-hostile), and press

. You will get a screen of the target’s inventory. There may be no items, one item, or multiple items. Assuming the character is carrying inventory, each item show a percentage within parentheses. This is the probability of being caught. Press

to pickpocket the item highlighted or the option to take all items. You can press

more than once to take specific items in inventory. No matter how many items there are in the target’s inventory, successfully pickpocketing them counts as only one attempt. However, if you pickpocket an item and then back out of the inventory screen, you can pickpocket the same target again and get credit for the second attempt as a separate pickpocket.
Pickpocketing while invisible is an easy way to rack up successful pickpocket attempts. You can become invisible by taking a phasewalk potion or employing the Finesse “Smoke Bomb” ability. To ensure success, employ the potion or the ability when the target is unaware. A minor phasewalk potion can be made with 2X Star Thistle, 1X Cripplespore Caps, and 2X Sky Blossom. It requires journeyman alchemy skill.
Riona Helt in Asker's Alley (Ysa) sells Greater Phasewalk Potion and
Gelphyne Nargyfier in Rathir - The Lower Tunnels sells Master Phasewalk Potion. The Finesse Smoke Bomb ability can be unlocked after investing 35 points in the Finesse class.
Another way of avoiding being caught is to pickpocket and backstab the target in quick succession. Assuming daggers are in the primary weapon slot, press

to crouch and approach a lone enemy or non-hostile. Press

,

,

when you see the prompt. The first

brings up the target’s inventory, the second

takes all items in the targets inventory, the

backstabs the enemy providing daggers are in the primary weapon slot. If daggers are in the secondary weapon slot, press

instead of

. Although this is a little extra work, enemies backstabbed will count towards the following achievement if not already unlocked:
It is possible to grind the achievement out early in the game. However, I recommend unlocking it in normal game play. You will pickpocket more than ten times during the
Travelers quest line as a required activity and there are other occasions where you will be pickpocketing as a matter of course. Accordingly, there is no need to force the issue.
You can track your progress in two ways. First, you can use the XBox One achievement tracker. Make it a favorite if you want to keep tabs on it. Second, you can press

-> Status -> Stats and scroll down to ‘Social’ where you’ll see, “Characters Pickpocketed” followed by the number of successful attempts.