Find two adult horses or two adult donkeys
Tame each one by riding it until hearts appear
Hold a Golden Apple or Golden Carrot
Feed one Golden Apple or Golden Carrot to each animal
Wait for hearts to appear
A baby horse or baby donkey will spawn
Use the same method to breed a horse and a donkey only if you want a mule
Feed a horse and a donkey a Golden Apple or Golden Carrot each
Wait for hearts to appear
A baby mule will spawn