In each island, collect 7 small Gems of each color and after beating the boss of the island, you'll get the big Gem. Do the same thing on each island to get the 7 big Gems. Once you beat the boss of the 7th island, instead of getting the bad ending, you'll proceed to a final 8th island.
At the title screen, push the following in this order: Left, Button 2, Right, Up, Down, Left, Right and Button 2. The words ''PRESS START BUTTON'' will be replaced by ''WORLD 1'', press button 2 to change the world number and button 1 to select the world.