I just scored an appointment in probably the busiest service Canada center in Ontario from my home country.
Go to this link:
https://eservices.canada.ca/en/service/
Fill out the form. Instead of choosing Outside Canada as your Country, choose Canada and then choose the province you are going to land in.
Give your home country number. They will still call you. Book one month in advance. Tell them the date of your landing and where exactly you will land.
The first time someone called they refused to give me an appointment and were very rude. The second time I filled out the form I was fortunate enough to speak to a sweet woman and she helped me book an appointment near the place I will be staying. Without the appointment, I would have to wait at least 6 hours in the queue. So be persistent, if it doesn't work out the first time, fill out the form again.