Overfly the threshold at 50 ft with an N1 consistent with your weight,config,wind and altitude.
Look at unreliable airspeed for an idea.
If you come in with too mych thrust ull float and underthrust ull sink.
Depending on your rate of descent,which normally should be around 700/800 ft/min..while looking at your aiming point,you will see it rise around 30 feet.
This is the time to gently start the flare by increasing the pitch gently and reducing the thrust towards idle while looking down the runway,,,as you have finished the flare,and your horizon doesnt move(no relative movement),you should have touchdown,if not release the elevator force and the aircraft will settle.
If u have difficulty with the visual in the beginning,once u pass the threshold,count 1,2 and start to pitch up gently and as u do it reduce the thrust as one will countrract the other...if u keep the thrust too long u will land long as the increase in pitch will turn into an increase of lift.If you reduce the thrust too fast,your sink rate will increase and you will need to pitch up more than necessary.
Read the fcom its really the way,,btw it states "appriximately" 20 feet as it depends on your seat position(if u sit too low ull flare too late,sit too high ull flare too high),descent rate...
If you start the flare at 30 feet,by the time the nose will rise the aircraft will about 20 feet..
Stable approach helps a stable pitch and thrust at 50feet and makes your flare maneuver stable..
Enjoy and good luck!!remember flaring is a visual maneuver.