How do you cook on HorseIsle?

You can't actually cook in the game Horse Isle. It's a horse-centric game, focused on raising, training, and caring for your horses. The game doesn't feature any gameplay elements related to cooking.

Perhaps you're thinking of another game?

If you're interested in horse-related games with cooking features, you might try searching for games with "horse" and "cooking" in the title or tags.