1. Take out your fishing pole
2. Add bait by hitting the 'Add Bait' button
3. Hit the space bar to launch your rod
4. Use the up and down arrows to adjust the length of line
5. Use the left and right arrows to rotate character
You will see on the upper left hand side of the screen an image of a fishing hook. When you adjust the line length and the rotation of your character, you might see a silhouette of a fish. Adjust the hook even further to attract it to the fish.
Then, once your fish is near the hook and ready to get hooked, press the spacebar
Tip: One of the areas that are populated with fishes is River Forest, the region with the castle.