Metro line 5 or 10, or RER C, to Gare d'Austerlitz, the garden entrance is 2 min away across the square. Also line 7 to Censier- Daubenton or Jussieu. This is a self-guided tour: no group, no guide waiting. Start whenever you like. Entry to the garden is free.
Finish at the last stop, the Flower market on "île de la Cité".