Instructions
Using a Mushroom House
Play the "New Super Mario Bros." story mode and enter any desired level.
Reach the end of the level, marked by a flag pole. Watch the timer in the corner of the screen carefully.
Run through the goal when the last two numbers on the timer are 11, 22 or 33. This causes a red Mushroom House to appear near the first level of the world.
Go to the red Mushroom House and select the box inside to open it. You are given a random power-up, which may be a Mini Mushroom.
Using Level 1-4
Go to the first world by tapping the "World 1" icon on the bottom of the screen. You do not need to do this if you are already in the first world. Enter the fourth level.
Progress through the stage until you see a small pipe below you. Hit the question mark box above you, and a Mini Mushroom pops out.
Grab the Mini-Mushroom to become tiny. Exit the stage, if you have completed it at least once before, by hitting "Start" and then "Select."