I'm a local Skywarn spotter for our locale and have a few points.
First of all, most people in a tornado who die are killed by flying debris or exposure to the winds themselves. They are struck by two by fours, walls, bricks, cars and appliances, not mention glass which acts like razor blades zipping along at 200 plus miles an hour. Any airborne particles and debris are bullets and missiles in the violent winds. People caught outside literally have their flesh sandblasted off of them by sand, dirt and pebbles.
Being crushed by falling walls, masonry and roofs are secondary causes of death in a tornado.
Therefore:
Basement or cellar or lowest possible floor of house is best.
Get under a sturdy table or bench also, under a staircase in the basement or lowest floor is good too. Better to be buried and be dug out after a time than killed outright by flying or collapsing structures.
The Southwest or corner of building the tornado is coming from is a MYTH. Oftentimes the swirling wind will dump an entire structure in the corners of a basement. Center of basement under a heavy table is best, even better under a table near any support corners or posts.
If there is no basement or cellar get into an interior room on the lowest floor of the house.
Rule of thumb is to put as many walls between you and the tornado - away from windows. Often you will see survivors in ranch homes where the interior bathrooms and closets are the only things left standing. Pipes in the bathroom add extra reinforcement - so getting in the tub is not a bad idea either.
ABANDON mobile homes and cars for cellars or a ditch. Cover head with arms and hands. The worst possible place to be is in a mobile home or car. My neighbor was killed in a tornado in his mobile home last year, his place was picked up before he even knew there was a tornado and it was slammed into a bunch of trees - trapping him in a mangled wreck that was on fire due to his propane tank spewing flame on the wreckage. He burned to death before we could get him out. His home was sent sailing nearly a quarter mile from where it was.
Leave a mobile home, car or trailer for a cellar, sturdier shelter, ditch or culvert.
Opening windows is also a myth. opening windows just provides the wind an opportunity to fill the house and peel the roof off or blow the walls out easier.
Getting behind masonry or brick walls is also very dangerous. High winds can topple a brick or masonry wall right on top of you and you will be crushed.
Never try to outrun a tornado ala the movie Twister. You will die. Either your car will become an airborne missile or something will smash into it. Cars are tossed around like tin foil even in weak tornadoes, and the remnants of cars after an F-3 wrapped around poles and trees are mangled far worse than any collisions on the roadways.
It is also a MYTH to get under a viaduct during a tornado if caught in your car. This is due a famous bit of footage in 1991 where a news crew and family in Kansas was hit dead on by an F-0 tornado - and they survived. An F-0 to F-1 are lightweight winds less than 120 MPH in comparison to the 125 - 318 MPH winds of many midwest tornadoes.
Many in Moore OK on May 3 1999 - were killed when they were sucked out of the viaducts and sandblasted to death. It's better to get BELOW ground - below flying debris to escape being struck by debris.
There are exceptions to every suggestion however, due to the unpredictable and ridiculously violent attributes of the winds. A multi-vortex tornado may contain two or more super violent funnels spinning around one another inside a larger funnel.
In large f-4 and f-5 tornadoes - where windspeeds are over 300 MPH - a prayer is sometimes the best protection. When I lived in Texas many years ago - a tornado literally scoured pavement off the road and sucked foundations right out of the ground.
ANY thunderstorm that grows to severe levels can produce a tornado. It does not necessarily need to be a Texas Supercell. Sometimes squall line thunderstorms can produce rapid and weak tornadoes.
My advice - we seem to be in a very active outbreak pattern. Get an NOAA weather radio and when storms are being forecast - keep an eye to the sky and an ear to the radio for watches and warnings. Seconds can literally mean life or death, and being ready to move to shelter quick when storms approach is the wisest counsel I can give.