Gather at least two Mushroom Blocks. Place one against a wall and pick up the other one. Run and jump towards the wall, toss the Mushroom Block just before you touch the wall, then land on the first Mushroom Block. Hopefully, your character is now overlapping the Mushroom Block you threw. Next, press B to pick up the Mushroom Block you're standing on, and then jump. The Mushroom Block you threw is now floating in the air! Repeat the steps above and see how high you can go.