Heel-toe shifting is done on downshift only, when slowing to enter a corner. It's only really done to select the proper gear under hard breaking.
Procedure is about like this:
As you are approaching the corner (hard on the brakes) you push the clutch in with your left foot and begin shifting.. but you have to match the revs! And your right foot is allready busy witht he brake pedal!
No matter - while keeping one part of your foot on the brake (which part will vary between vehicles, but we'll say the ball of your foot for this example) you move the heel of your foot over to the gas pedal and blip the revs, complete the shift, let off the clutch and don't upset the balance of the car while entering the corner.
I hope I explained this correctly - it's much easier to just demonstrate.
