1. Find a throwable item.

Mystical Banana:

You can find the banana in the Arcade Room next to the Snowball Skirmish game.

Pumpkin:

You can find the pumpkin in the  Canvas Tent in Sub Alpine Forest in the boiling pot.

Snowball:

You can find the snowball in Webo Ridge near Snowball Skirmish. (Note: Currently it cannot be acquired, but it will be fixed in time.)

2. Click the icon of a throwable item in the menu on the right.

3. Aim and throw the item.