Speak to Darwin the Banker in Seers' Village bank and he will say there was a thief that stole everything including a person's christmas cracker and will ask if you can get it back. Inspect the users' chest to view his bank, view through it and withdraw a santa hat with fingerprints on it and give it to the chemist in Varrock. Speak to the chemist again to retrieve it and he will say these are the fingerprints of Darren Lightfinger from the Thieves' Guild. Go to the Thieves' Guild in Lumbridge and speak to him about the stolen christmas cracker. He will get angry and take away your position in the guild. Now you will have to pickpocket off him but first you will need to know how to pickpocket a master pickpocketer.