The tour starts from the Goddess Diana fountain in Green Park, next to the Colicci refreshment stand. Enter Green Park via the entrance gates next to the Ritz Hotel.
If taking the underground, disembark at Green Park Station. The Green Park/Buckingham Palace exit will lead you to the fountain.
The tour usually ends at Trafalgar Square, however occasionally it ends at Parliament Square. Ask your guide at the beginning of the tour for the exact end location.