First find the book on the catalogue.
Once you find the book that you want on the catalogue it will show you the classmark and also which floor the book is located on. Make a note of the classmark and make sure it says that a copy is available.
Floor plans can be found at the entrance to each floor, use these to find the general area you need.
Shelf end guiding will help you find the right row. The books are arranged on the shelf in classmark order from top to bottom bay by bay.