Start by going to the Olympic Games and enter Olympic Mode. Select the Long Jump event on High difficulty. Follow along through the tutorials until you get to the actual event.
To run down the track, alternate pressing the "A" and "B" buttons as fast as you can. The more speed you can get, the better your jump will be. Before reaching the end of the track hold down the "LB" button and release it when it hits the green area of the gauge (approx. 45 degrees). Once you are in the air, you will receive a random series of button prompts that will extend the length of your jump. You need to do these correctly, and as quickly as possible. The more correct button presses you can make before hitting the ground, the longer your jump will be.