Spoilers for Act 3, by the way.

1. Get to Act 3 and start the Murder Tribunal Quest.
2. Separate your party to have the main thief be separated and invisible.
3. you want this character’s sleight of hand to be buffed to the max, Smuggler’s Ring, Gloves of Power, Graceful Cloth/Enhance Ability (Dex), Guidance, Bardic Inspiration, Proficiency, Expertise, Shapeshifter’s Boon Ring, etc.
4. Speak to the Bhaal Cultists and seek judgement from the Tribunal.
5. Once the spirit is summoned (such as Dribbles the clown) you can move your invisible thief up to them and pickpocket them.

It is a DC 29 check, so going Rogue 10 for Reliable Talent and having +8 proficiency is only going to get you to 18. Smuggler’s Ring(+2), Guidance, Bardic Inspiration, Gloves of Absolute Power and Shapeshifter’s boon Ring give you a minimum of +1. Minimum total increase of +6, so we’re at 24 minimum now.

Graceful cloth gives another +1 Dex modifier, and you can max you Dex with ASI, giving you another +5.

In theory you don’t need all these items, as Reliable Talent gets you a minimum 30 with all these items.

Be warned, as after the thieving is done, combat will start.