When should I begin reducing my airspeed while on final approach to landing?
You should begin reducing your final approach speed at the start of the roundout which, in a normal descent, begins about 10’-20’ about the ground. The round out is a slow, smooth transition from a normal approach attitude to a landing attitude. During the roundout, apply gradual back pressure while reducing power and slowly increase the pitch attitude to reduce your forward energy (airspeed) for a shallower descent rate as you approach the landing pitch attitude and airspeed.
The round out is executed at a rate such that the proper landing attitude and the proper touchdown airspeed are reached at the same time just as the wheels contact the landing surface. Touchdown speed should be at or just above the approximate stalling speed. A common technique to making a smooth touchdown is to hold the wheels just a few inches off the ground as long as possible using back pressure while the power is at idle.