Advance polls are open and apparently, they will be until May 28. That's pretty awesome. I always think it's best to vote in advance so if there are any problems, you have time to resolve them beforehand. Note that, just like with election day, you
need to go to the voting poll assigned to you (which is usually a different location than on election day)
OR go to the local returning office in your area to vote. You can do the latter any day during operating hours.
My wife and I will be voting by mail. We haven't received our kits yet but should soon. Then I can either drop them off or just bring them to the local returning office. You can apply to vote by mail up until May 27.
It's best not to leave thinking about voting until the last minute. Check to make sure you're on the voter's list. If you didn't get a card, you should definitely check and save yourself a possible phone call. But you can vote on election day even if you're not on the list. It just makes things more complicated, mostly for the poll clerk helping you.
More info here:
https://www.elections.on.ca/en.html