From Tiny Acorns

The first caper of the thieving guild was released on the 28th of April 2010, and requires you to steal a toy dragon made of rubies. Talk to the Guildmaster (Darren) to start.

Walkthrough
After obtaining the caper from Darren, get to varrock square. Talk to the dwarf by the crafting stall and then the guard and ask them every possible question. The guard will tell you about the dwarf being obsessed with his talisman. Now pickpocket the dwarf.

Stand north of the dwarf, and left-click the talisman to put it on the ground a small distance awy (you'll automatically walk to do this). Now quickly run to the guard, and when the dwarf walks to the talisman, distract the guard (you will tell him about Romeo looking suspicious). When both the guard and dwarf are distracted, quickly steal from the stall. Now talk to the dwarf, to get the bank notes and return it to Darren after that. Congratulations

Trivia
It is possible to reobtain the talisman after this caper is complete, but for no particularly obvious reason.