[blue00gt]

What you described is basically correct, although you do not have to let the clutch out slowly once you have practiced enough. in order to properly do this you use your toes on the brake and 'blip' the throttle with your heel/side of your foot, hence the name 'heel and toe shifting'. The best way to practice is when you are braking in a straight line for a stop sign or stop light. The lower gears are hardest to do smoothly. You will know when you've got it right because you won't feel like jerking forward in your seat when you let the clutch out.
The whole idea behind it is to prevent loss of traction at the drive wheels and reduce wear on the clutch by matching the engine speed to the vehicle speed so that the tires and clutch don't have to make this happen. This is especially important when downshifting just before corners because a bad late downshift can cause you to spin out.
As a side note, on the 99+stangs the gas pedal is much lower than the brake, making it a lot more difficult to pull off without your foot slipping off the brake. There is a mod that you can do by adding a spacer in your throttle cable to raise up the gas pedal. The install is described on mustangworld.com. This can also give you more available throttle as it is rumored (I have no way of knowing if it's true or not) that on some 99+ GT's the carpet prevents the pedal from hitting full throttle.
Hope that helps and happy Heel and Toeing.