Flare at 20 FT. Hold the pitch attitude until the main wheels touch down. When you're a lot more experienced, you can get away with flaring a little later. Close the thrust levers after you have started the flare and reduce the thrust gradually, so the levers are closed just before touchdown, i.e. Don't slam them closed. Don't close them before the flare or you'll bang it in.
As for the stablised approach, keep the touchdown zone in the same position in the window and keep the rate of descent constant until the flare.
30ft is too high, and you'll thump it in. 20 FT is the number for learning the ropes.