747 bus
The closest stop to the residence (at Rene Levesque and Guy) is currently cancelled because of construction, so get off at Rene Levesque and Bishop. This will be the second stop---but see below.
When you take the 747 bus from the airport, you need to buy a ticket on the arrivals level inside the terminal.
Head to your right as you exit customs and you'll see machines and maybe a person to help you near an STM sign. Choose a non-rechargeable 24hour card for $11. You can use the card on metros and buses for 24 hours. (You can buy another for your return trip to the airport.)
Exit the terminal and head towards your right to the waiting area (about 100m)
Take a 747 bus that follows the LONG route to Terminus Gare 'd Autocars de Montreal. The first stop will be after a long ride, at the Lionel Groulx metro. Get off at the second stop at Rene Levesque and Bishop and head back three blocks to Guy, where you'll see the Grey Nuns Residence.Â
If you accidentally get on the SHORT route bus, the only stop is at the Lionel Groulx metro, where you will get off, use your ticket for a free transfer to the metro system and take the orange line towards Montmorency for two stops to Lucien L'Allier, where you will exit, turn right up the hill, and find yourself on Rene Levesque Boulevard. Cross Rene-Levesque, turn left and head towards Guy.