As the thugs are standing outside of her front door, which makes it somewhat difficult to do it non-lethally. She gives you the key to her front door and asks you to kill them. Your first side mission for Granny Rags involves getting rid of three 'gentlemen callers,' which are three members of the Bottle Street Gang. You'll see her throwing things off of a balcony, so go there, climb up onto the balcony she was on and you'll find her downstairs. Granny Rags is located in the first area you come across, in the alley to the right. Completing the two side missions will open the way to Granny Rag's final side mission in mission 7: The Flooded District. Completing both side missions will reward you with two Runes. Granny Rags has two side missions for you to complete in Mission 2: High Overseer Campbell, which have no bearing on your main objective in this mission. ! PLEASE READ: The final side mission for Granny Rags is ONLY accessible by completing her first two side missions (see below) and Slakjaw's first two side missions (see the achievement: Street Conspiracy' before you get to mission 7: The Flooded District, otherwise the thug you come across in the sewers in mission 7 will not be there and as such you will not be able to complete either the 'Gentlemen Callers' or 'Street Conspiracy' achievements.

Dishonored 2 mission 7 3rd rune