Get to world 3-1, and become Super Mario. At the end of the level(the staircase before the end flag) you will see two turtles coming down the stairs, jump over the first one and then jump on the second one. It should hit the star and bounce back, from here jump on it and keep yourself up, after a certain number of bounces without hitting the ground, each consecutive bounce will get you a 1-up. Get as many as you want but be warned, if you get over 100 you will get a Game Over.