Use a rain gauge. Lawns need 1.0 to 1.5 inches of water per week. Apply the needed difference. Tuna cans work just fine.
Don't water everyday. Twice a week is fine using 1/2 to 3/4 inch of water each time. This is important in sandy soil to prevent nitrate in fertilizer from leaching below the root zone.
Water slowly and deeply so water soaks into soil. Avoid runoff; it wastes water and contaminates nearby water bodies!
Water in the morning. Hours between 6 am and 10 am are best. Avoid night time irrigation; it encourages fungal diseases.
Let it go dormant. Most perennial grasses have the ability to go dormant during dry spells; that is, they turn brown and stop growing. Such grass is very much alive and can survive for weeks until moisture returns.