Having finished Glowenglow Estate job, you need to talk with Brynjolf. He will tell you that Maven Black-Briar has asked for you by name. Head over to the Bee and Barb. Just inside, turn to the left and go up the stairs. At the top of the stairs you will want to go to the right. Sitting just around the corner is Maven Black-Briar. She will explain that she needs you to deal with her compeition: Honningbrew Meadery, run by Sabjorn. She sends you to her contact, Mallus Maccius
Make your way back to Whiterun. Travel over to the Bannered Mare. Once inside, head over to the left. You will find Mallus Maccius. He will tell you, if asked, why he wants things to go the way they are about to. He explains that he's set things in motion for Sabjorn to lose his meadery. The local captain is going to visit for a mead tasting. He wants you to ensure the Captain's poisoned with the pest control. Now you need to head over to Honningbrew Meadery.
Head inside the Meadery's main building. You will find Sabjorn. He is not very happy with the Dead Skeever on the floor nearby. Talk with him and accept the job to clear out the pests. You can Persuade or Intimidate him into giving you half the payment up front (500 gold) to deal with the Skeevers. You will not get "paid" for this portion of work if you do not. Sabjorn will give you the Pest Poison and the Honningbrew Basement Key.
Head over to the right and go through the door. Head forward a bit then turn to the left and go down the stairs. This will lead you to the entrance to the Honningbrew Basement. Go through the door and be mindful of the bear trap sitting just in front of it on the right.
Inside the basement, you are greeted by a Skeever Corpse. Turn to the South and get ready. You will be rushed by a few Skeevers and some Venomfang Skeevers. Head to the South after dealing with them. Turn to the West as you get to the back part of it. You will find the corridor to continue forward there. There is a pair of bear traps just in front of it. Be mindful of them as you start down the passage.
This next room has 4 smaller Frostbite Spiders and one large Frostbite Spider that you have to deal with. It is very easy to use the nearby hallway to keep them at bay and keep it to just one or two coming after you at a time. The large spider cannot enter the hallway, through it still can spit at you. Head through the spider room to the South after that. Turn to the East at the end of it. Go through the corridor to the next room. Head through that room carefully. There are at least 2 Bear traps lain on the ground.
When you turn to the South and see the next corridor, stop. There is a trip wire at the entrance of the tunnel. Activating it causes a swinging mace to trigger from overhead. As you start down this corridor, SAVE! The next room can be very difficult.
This is the last room of the basement. The problem is not the mix of Skeevers and Venomfang Skeeversv, but the hermit hiding in the back part of the room. Make some noise to lure in the nearby skeevers. Take them out in the corridor you are in. This leaves you alone to deal with the hermit: Hamelyn. He will be throwing very powerful Firebolts at you. If possible, attack him from a distance with your spells or arrows.
Defeat him and then look nearby the Alchemy Lab. Next to the lab is a chest. On top of that you can find a copy of Three Thieves. Take a moment to read it before you move onto the skeever nest. You will find a Skeever nest you were sent in to destroy. Interact with it and the Dragonborn will use the Pest Poison to eliminate the skeevers. One more stop to make.
Head to the East and through the corridor. This will lead you directly to the final destination you are after. Be careful as you enter the building because there is another bear trap sitting on the ground. Go over to the door to the Southeast. This leads into the Honningbrew Boilery.
Inside the Honningbrew Boilery, go to the opposite end of the room and turn to the left. Head on up the stairs there. Go around the walkway to the opposite side and the marked barrel.
Having dealt with the Skeevers and their creator as well as poisoning the Mead, head back to the main building. Be sure to grab the key to the right of the door leading back to Skyrim. Head over to the left from there and to the Meadery.
Inside the Honningbrew Meadery you will find Commander Caius talking with Sabjorn. Talk with Sabjorn about completing the job. he will refuse to pay you until after that tasting. Just let things play out now. Sabjorn will be arrested and Mallus Maccius will be put in charge of the Meadery.
Head to the East, then turn to the North. Jump over the barrels (or go around them) to reach the stairs going to the walkway above. Follow them around to the far side. There you will find the entrance to the room up there.
Now go to the door immediately right of that chest of drawers. This door is an Expert level lock. Having a lot of picks is very helpful. Open it up and go inside to the right. On the table there you will find the Honningbrew Decanter. Grab it to sell later to the Thieves' Guild. Clear out of the room, grabbing the coin purse sitting on the floor to the right of the table. There is another coin purse behind the door you open. Leave the Meadery and head back to Riften.
Go to where you met Maven Black-Briar. She will be wherever you talked with her to start the quest. She will not be happy about the note and recommend that you bring the note back to the Thieves' Guild.