The J-Stroke is a version of the Forward Stroke that gets used by both solo and tandem stern paddlers because it's the most effective way to keep your boat going in a straight line while keeping your momentum.
Using regular Forward Strokes you'll notice that your canoe turns a little to the off side with each one you take. The J-Stroke fixes this issue by adding a small pry at the end of power phase of the Forward Stroke. Without it you'll probably find yourself switching sides with your paddle or taking aggressive pry strokes which may correct your course, but they'll also slow you down.
The J-Stroke starts with your regular Forward Stroke, but instead of finishing it normally, you'll start a pry when the shaft reaches a point beside your knee. This pry corrects for any turning momentum created by your Forward Stroke.
To initiate this pry twist your control hand so that your thumb is pointing towards the bow of the boat. At the same time allow the shaft to rotate in your other hand by slightly loosening your grip. With the power face of your paddle now facing away from you, pull your control hand inward and over your onside knee while bracing the paddle against the gunwale with your shaft hand. This inward motion will pry your blade off the gunwale and away from the canoe. With your course corrected, you're now ready for your next Forward Stroke.
Although it will take practice, before long you'll find J-Stroke blends from one Forward Stroke into the next; this will allow your canoe to carve in a straight line across the water.