Reach Rue Jacob from each airport is rather simple.
Click on the plane to see the way to do it !

 

If you don't carry too many or too heavy luggages ... otherwise, use a cab !!!

 

from Orly Airport :

Use'OrlyBus to station "RER B Denfert-Rochereau" (terminus: last stop)
Then, choose RER B direction "Aéroport Ch. de Gaulle - Mitry Claye", get off at "Châtelet les Halles" Station
Now, get in the Metro ligne #4 direction "Porte d'Orléans", get off at "Saint-Germain-des-Près" station.
Choose "Sortie Place (or) Eglise Saint-Germain des Près", climb the stairs, then turn right, straight on (Eglise St Germain des Près stands on your right) leave first street on the left (rue de l'Abbaye), at first traffic-light, turn left in rue Jacob.
or:
Use the "navette" (bus) to "RER C Pont de Rungis - Aéroport d'Orly" station
Take RER C direction " Versailles Rive Gauche, St Quentin", get off at "Saint Michel" station
Take Metro ligne #4 direction "Porte d'Orléans", get off at "Saint-Germain-des-Près" station.
Choose "Sortie Place (or) Eglise Saint-Germain des Près", climb the stairs, then turn right, straight on (Eglise St Germain des Près stands on your right) leave first street on the left (rue de l'Abbaye), at first traffic-light, turn left in rue Jacob.
or:
Use OrlyVal (train) to Anthony (ternminus)
Take RER B direction "Aéroport Ch. de Gaulle - Mitry Claye", get off at Châtelet les Halles station
Take Metro ligne #4 direction "Porte d'Orléans", get off at "Saint-Germain-des-Près" station.
Choose "Sortie Place (or) Eglise Saint-Germain des Près", climb the stairs, then turn right, straight on (Eglise St Germain des Près stands on your right) leave first street on the left (rue de l'Abbaye), at first traffic-light, turn left in rue Jacob.

from Roissy Charles de Gaulle:

Take "RoissyBus" to Opéra station (terminus: last stop) go by feet to "Auber" station (short walk)
Take "RER B" direction "Marne la Vallée - Boissy St Léger", get off at "Châtelet les Halles" station
Take Metro ligne #4 direction "Porte d'Orléans", get off at "Saint-Germain-des-Près" station.
Choose "Sortie Place (or) Eglise Saint-Germain des Près", climb the stairs, then turn right, straight on (Eglise St Germain des Près stands on your right) leave first street on the left (rue de l'Abbaye), at first traffic-light, turn left in rue Jacob.
or:
Take "RER B" direction "Robinson, St Rémy les Chevreuses", gett off at "Châtelet les Halles" station
take Metro ligne #4 direction "Porte d'Orléans", get off at "Saint-Germain-des-Près" station.
Choose "Sortie Place (or) Eglise Saint-Germain des Près", climb the stairs, then turn right, straight on (Eglise St Germain des Près stands on your right) leave first street on the left (rue de l'Abbaye), at first traffic-light, turn left in rue Jacob.