How could I increase horsepower on my 2011 Ford Focus SE
Put a Mustang badge on it ;)
short of violating emission standards, be sure the fuel filter and Oil filter are fresh, exhaust runs at standard backpressure and the timing (automatic, these days) should be correct~
Remove the radiator cap and drive a Mustang GT under it.
Or the tried and true method of dropping it out of a C130 at 30,000 feet.