Find a horse in a plains, savanna, or similar biome
Make sure your hand is empty
Right-click the horse to mount it
Keep getting bucked off until hearts appear
Repeat mounting the horse until it stays calm
Open the horse’s inventory while riding it
Put a saddle on the horse
Use a lead if you want to move it easily
Feed the horse apples, wheat, sugar, hay bales, or golden apples to help tame or heal it