I don't know about estimating ahead- I've never had this done here in Ontario. I simply do my taxes for the year and send it off. When I get the notice of what I owe I call the number on the back of the form like 5 Little Monkeys said. They've always been really helpful. You just ask to make payment arrangements and they even ask me how many months I'd like to pay the amount owing over. With interest of course!