Double clutching is depressing the clutch, shifting out of the gear and into neutral, releasing the clutch, giving the engine a squirt on the accelerator - or not depending on your technique, then depressing the clutch again and selecting the next gear. Double clutch - as in you do the clutch action twice.
Heel and toe is used on down shifting while braking, right foot on the brakes, then you want to change down a gear at the same time. So you put use your left foot to depress the clutch, then with you right toes still riding the brakes swivle your foot to give the accelerator a squirt - and try to match the revs to what they will bein the next gear, with your heel whilst changing gears, then release the clutch...