Note: this short-term forecast tool allows you to check whether northern lights will be visible in the coming hours or are visible right now (assuming the skies are dark, and there are no clouds)
Step 1: Check your local cloud cover forecast
Clear Sky Charts provides real-time data about expected cloud cover in different places. Find a place near you, and see what time is the best for sky-watching. Here is the data for David Dunlop Observatory in Richmond Hill (the more white, the more clouds there are):
Step 2: Get away from the lights, if you can
Above is the light pollution map from Dark Sky Finder. Big cities look depressingly bright, but it is not the end of the world - just get away from lights as much as possible - even a city park or a football field will do.
Step 3: Find Clear View of Northern Horizon
They are called Northern Lights for a reason, and the you only see them overhead in places like Yukon, Alaska or Finland. If you live as far south as Southen Ontario, your best bet is to look for them in close to the northern horizon. If you live in Toronto or Ottawa your best bet may be a window of a high-rise with northern exposure, or an edge of a big field (a park or a football field).
Step 4: Didn't see anything? Sign-up for Aurora Forecast Alert