This is the best picture I found
This technique does take practice. It's not just about blipping the throttle, you have to know how much to blip it and that depends in which gear you are and how many rpms you have dailed in, and how many rpm you need for the next gear... After you learn it, it all becomes second nature and you dont have to think about anything.
What you do with the heel-toe is trying to match the speed of your transmission with the speed of your engine(or more precisly the flywheel). When you are driving at 100km/h in 5th gear at 2500RPM when you downshift to 4th gear this same speed will be achived at 4000rpm. So as you see you need to blip the throttle in order to reach the 4000RPM that are needed for the lower gear. Now this applies to braking also, because while you are braking the RPMs drop and so does the velocity, when you trail brake or just brake in straight line exactly the same situation happens except your working with smaller numbers.
A little tip, when you heel-toe to lets say 2nd gear or even 1st blip the throttle a lot since these gears are very short on most street cars.
Hope that helped.