Visiting Triss's home
Triss Merigold will be your contact in Novigrad. Her house is located in the center of the city (M6,16). However, after you get through the entrance gate, the only people you will find will be thieves - the sorceress isn't there. You can enter the house and search through the desks but the clues you find won't tell you much (although finding the Rose of Remembrance will allow you to talk about it later). To find Triss you must use the help of proper city "organizations". You will have to reach the Putrid Grove. Thieves or beggars will lead you there.
Option A: Beggars
Talk to the beggar sitting on the wooden scaffolding above the road (M6,17). Keep talking to him until he gives you the password: Old sow's farrowed piglets. If you ask for the grove directly, he won't tell you anything and ask you to come back later. Do it. Go to the entrance gate (M6,18) to the location and give the password - if you make a mistake, you will have to pay 50 crowns or use Axii sign (additional experience points).
Option B: Thieves
Find the thief on the market - just stand in the circle and wait. Follow your target to the hideout - you won't be able to tell the password, so you can pay or use Axii. But if you walk too close and remain visible for too long, the thief will find out that you're following him and will start the conversation. If you tell him to lead you to the Grove, he will lure you to a trap and you will have to defeat 4 enemies at once in a fist fight. If you manage to do it, you will be lead to the target and you will receive the password (Old sow's farrowed piglets). You can decide not to tell the password - in that case, you pay 50 crowns or use Axii (additional experience points). The beggars path is easier and quicker.
King of Beggars
Now go to the meeting with the King of Beggars (M6,18). After the conversation follow Triss. Soon you will reach the sewers. Kill the drowner and search for a button that opens hidden passage - with the use of witcher senses you will find it on the pillar.
Soon you will reach a bridge. You must dive in order to find the missing bag - you will find it in front of another bridge at the bottom of the river. Return to Triss with the package. Now you must go to sorceress' employer. You must clean the warehouse of the rats, luckily with the use of magic. All you have to do is place three rat traps in three corners of the room, at the places where holes in the walls are visible. After completing the quest and a short talk, three temple guards (level 10) will attack you. Deal with them. You don't have to worry about your colleague - she can take care of herself. Leave the warehouse, after a short cutscene the quest will end.