When:
Any time during the game, but easiest to get done right at the start against the first enemy you come across.
Method:
Head to the north of where you start to the next island, the one with the campfire. Look for small stones along the beach - you can pick them up with the action button. When you head north again you'll come across the game's first enemies: Crabs!
In the turn based combat that follows, when it's your turn to act, choose the "item" option and select the stone from your inventory to use it. You'll throw it at the crab for minor stamina damage.