You just right click with it or use it to make a cake.
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You use a bucket on a cow.
Right click a cow with a bucket.
You can not find milk in Minecraft. Rather, you must craft a bucket and use it on a cow's nipples.
Place the bucket in your active inventory slot, and hold the right mouse button.
To fill up a bucket in minecraft, there are two ways to do it, Go to a water source, aim with your cursor with the bucket in your hand and right click, this will cause the water to leave where it was and go into your bucket. Using the same technique as above, aim your bucket at a cow and right click and it will fill it up with milk.