Apply enough sunscreen to cover all exposed skin. A good rule of thumb is a teaspoon for the face, and 1 ounce to the body (tip: 1 ounce is roughly the size of a shot glass).
Don't forget to apply to commonly overlooked areas including the tops of feet, neck, the back of the neck, ears, the backs of the knees, and the top of the head.
Sunscreen takes approximately 15 minutes to absorb into skin and provide effective defense, which is why you should apply 15 minutes before sun exposure.
When outdoors, reapply sunscreen approximately every two hours, or after swimming or sweating. You can always check the directions on the bottle for clear usage directions.
Lips need protection, too! Try a lip balm or lipstick that contains sunscreen with an SPF of 30 or higher.