examples on when to use heel-n-toe and double-clutch:

double-clutch:
say your cruising around town on 5th gear (70km/h) and the light ahead of you hits yellow and your not really close enough to just breeze through it, and being in 5th doesn't give you enough torque. So in this case I would double-clutch into 3rd and floor it to get enough *umph* to get you through the light.

heel-n-toe:
This is different because you will be needing to use the brakes. So say your coming to a turn and your in 4th gear, so you would need to slow the car down somehow so quick way to do is to heel-n-toe into 3rd then to 2nd and then mash the gas on the way out of the turn. Heel-n-toe is when you use both feet and all pedals at the same time. So easy way is to brake, then step on clutch and while stepping on clutch you use the heel of the braking foot to blip the gas, then downgear and and release the clutch all in one motion. You can either stay on the brake or release doesn't matter.

hope that helps. Best Motoring videos are best to show these manuvers.