Move down to the lowest level in your home. Tornados only stay on the ground for an average of 20 minutes. During this time, they can rise and come down over and over. This means that, even if you don't have a basement, the safest level in your home is the ground level.
Place yourself into an interior room if you do not have a basement. This could be a closet if nothing else. If you do have a basement, move under the stairs. Stairs and the stairwell are extremely sturdy. This is one of the safest places in your home.
Crawl under a heavy table or hold a large book over your head. This is to protect your head and neck from flying debris. It is possible the roof could collapse. This is why a sturdy table is recommended.
Surround yourself with couch cushions. Try to create as many barriers between you and the tornado. Couch cushions will soften any blows flying debris could cause to your body.
Hold on to something secured to the ground. Tornadoes move at incredible speeds. This makes the pressure inside a tornado very low and low pressure creates sucking. If a tornado charges over your house, the only chance you have of staying in place is to hold on tightly to a secured object. This could be the stairs, floorboards, or anything else that looks sturdy.