After completing the Loud and Clear quest, Brynjolf will reveal that Maven Black-Briar requested your presence. Head up to Riften either through the Ratway, or through the new South-West exit from the Ragged Flagon-Cistern that leads to the Riften cemetery. Maven can be found in either the Bee and Barb, or in Black-Briar Manor. The conversation choices make no difference in the end; you will learn of a job that Maven has for you regarding her one and only competitor, Sabjorn of the Honningbrew Meadery near Whiterun.
Maven wants you to head to Whiterun to meet with her inside man to set into motion a plan that will shutdown the Honningbrew Meadery. In Whiterun, follow the road to the East, enter the Bannered Mare and locate Mallus Maccius; to learn of a tasting that Sabjorn is preparing for the Captain of the Guard, and the fact that the showing is being interrupted by a skeever infestation. Mallus plan involves your “helping” Sabjorn clearing out the infestation and poisoning the skeever nest (as well as the Honningbrew Meadery’s vats).
After you have finished speaking with Mallus, head over to the meadery, which now should be marked on your map; if it is not, simply follow the river South-East of Whiterun until you reach Honningbrew. Enter the establishment and speak with Sabjorn about his skeever problem, and either persuade or intimidate him to receive 500g otherwise you won’t receive any payment from him for the “job” you are about to perform.
After this conversation, head into the other room and enter the basement; watch out for the bear traps and skeever running around in here, and work your way through the tunnel located behind the fallen bookshelf. Eventually, you will come across a tunnel with a tripwire trap, be careful because there is a particularly difficult in the clearing at the end of this tunnel. Kill him, read his journal to see just how psychotic he was, and keep pressing along.
The tunnel next to the alchemy table leads to the basement of the Honningbrew Boilery; enter here and climb the wooden steps to the second floor. Walk around to the Brewer Lid and activate it to poison the vat. Now, either exit the way you came or snag the key that is hanging next to the door and exit the building. Head back into the meadery and inform Sabjorn that the job in complete, and watch the results of your actions take place.
Afterwards, speak with Mallus to get permission to look through Sabjorn’s books, head into the other room and head upstairs; follow the upstais balcony around until you reach the door leading to Sabjorn’s former room. Using the key Mallus gave you, open Sabjorn’s dresser and retrieve the Promissory Note.
With note and decanter in hand, travel back to Riften, inform Maven of your success by giving her the Promissory Note, and she will direct you to take the note to the Guild (after rewarding you for a job well done of course). Turn in the Honningbrew Decanter to Delvin in the Ragged Flagon, then speak with Brynjolf to complete this quest and begin Scoundrel’s Folly.