Instructions
Enter a level and complete it by reaching the "Exit" sign. Most levels are played from left to right. As you navigate the level, make note of its layout as well as barrels and friendly animals.
Revisit each stage and search for bonus rooms. Bonus rooms are found in a variety of ways, including using barrels to break open a secret passage, using animals to reach certain areas and jumping off cliffs to find off-screen "transportation" barrels.
Continue this process until you find each bonus room. You will know you have found them all when your save file has "101%" next to it.