From Roissy-Charles de Gaulle Airport

Take the RER B at Aeroport Charles de Gaulle 2 TGV direction Saint-Rémy-les-Chevreuse (or Paris) until St Michel Notre Dame.

Take the Metro line 10 at Cluny la Sorbonne (there is a walking connection between RER St Michel Notre Dame and metro Cluny la Sorbonne) direction Gare d'Austerlitz until Jussieu. Walk along the rue Linné-Geoffroy St Hilaire.

It will take you between 1h and 1h30.

For more details see Paris public transport website RATP.