OK boys and girls. Here is how it is done. The expression heel and toes comes from the 20's and 30's on racing barges like Bentleys, 'cos the throttle was in between the clutch and brake, eeek!
Ok, you are ready for a downshift under braking. While braking with you right foot, swing your foot a bit anti clockwise, as you press the clutch you shift into neutral, release the clutch, and still braking give the throttle a blip with the right part of your foot. As the revs start to drop you press the clutch and shift to the gear you want. Swivel foot, let the clutch and brake go and floor the throttle.
Take this from an old git whose first car had no synchro, by design....
Double de clutching is the same without the braking bit. I mastered it at 17. It's very easy.