This is the last part of the optional quest Granny Rags gives you. You have to put the rat viscera you took from Dr. Galvani into the Distillery. This is fairly straightforward and you can easily avoid most of the guards:
There is a visual version of this guide at the end of this guide in the form of a YouTube video.
When you enter the Distillery you are in an area with multiple guards in it. Just go up onto some pipes and follow the all the way to the entrance of the distillery:
Jump onto the platform. You can ignore the guard standing on it. You now enter a storage area. The distiller is underneath the stairs. You can get into it through a tunnel to the right of the locked door: