To perform this glitch, you need to have at least 1 item in your use inventory, otherwise you can't access your use inventory menu.
Performing this glitch is different, depending on how many items you have in your use inventory menu.
1 item in Use inventory: Touch the screen around the middle of the right side of the black space.
2 or more items in Use inventory: Touch the small space between the top-right item and the edge of the screen.
Doing so, will make the item description say "Restores 80 HP"
Press "A" at this point to "Use" a potion.
NOTE: If you have any normal potions they will be consumed, but if not, you will use a potion that doesn't exist. High and Super Potions will not be consumed.