A Guild of Our Own is the third and final caper Guildmaster Darren Lightfinger will assign a player who has completed the Buyers and Cellars quest and the previous two capers, From Tiny Acorns and Lost Her Marbles.
Location of the Guild Registry
Find your way to east Ardougne. The Guild Registry is located in the south eastern section of the town. Enter the guild. Talk to the Registrar.
Pickpocket the Registrar to obtain a Cuckoo clock key to the cuckoo clock.
The next bit you have to do quickly! Read it before you proceed.
Location of the clock, lever and trapdoor
Click on the ornate cuckoo clock to wind it and run back into the main room with the guard. You must stand next to the counter, not by the doorway to the cuckoo clock room.
As soon as the Registrar is in the cuckoo clock room, lure the guard, knock him out, and then steal from the Guard to obtain the vault key.
Run as quickly as you can, behind the desk in the main room and pull the lever on the wall to open the trap door. Quickly climb down the trapdoor to enter the vault room.
If the Registrar returns to the main room before you enter the basement you will have to repeat the entire process. It may take a few tries to succeed.
When you succeed in getting down the trapdoor, the Registrar will ask Bernard what happened and then lock you in the vault to "stew for a while".
Pick the lock on the gates to enter the area with a large chest. (This may take many attempts, even with a lockpick.) (If it's not working, try using the lockpick on the door)
Check the Registry Strongbox to look for traps (otherwise you get hit for up to 3300 life points).
Open the chest to obtain the bonds.
Inspect the shortest box outside the vault area (under a vent in the wall) and exit.
Bank the vial of stench (a), as the Guild Registry cannot be re-entered with it.
Re-enter the Guild Registry and speak to the Registrar to complete the Thieves' Guild registration.
Return to Lumbridge and speak with Darren Lightfinger to complete the miniquest. Congratulations, caper complete!
If you talk to the guard before winding the cuckoo clock, he will say that the stench is vile. This is a play on vile being a homophone of vial, which in turn references the vial of stench.
Completing this caper is a requirement for the Completionist Cape, as a piece of music cannot be unlocked without it.
It is impossible to enter the Guild Registry after stealing the bank notes, as the player doesn't want to "tempt fate by going in there."