How do you use potion in The Skyward Sword?

Answer 1

equip it instead of your shield. if you have the option to catch it ( i've only tried w/ health fairies) it will indicate so on the screen. then press A. you may not catch it the 1st time but link will actively try to catch it. if that does'nt work, try the insect net.

Answer 2

You hold down the - button & release on the potion you want to use, the press A to drink it.