Best way is to take a slow car and practise.
Drive a Mini on Rome Circuit for instance.
Downshifting is depending on the revs your engine is making.
Suppose your mini goes into the red at 6000 rpm.
Situation 1:
You drive towards a 2nd hand corner whilst in 4th gear at 5900rpm.
Proper way is to start braking first, wait a second for the revs to drop, then start downshifting twice slowly.
Situation 2:
You're driving towards that same 2nd hand corner but now you're doing 3000rpm in 5th gear.
This time you can start braking and downshift almost simultanious because the engine was not revving at max rpm.
To answer your question more specific: how fast you should downshift is depending on how good your car is. In a FGT you can downshift really fast because the car brakes really fast. If you have a heavy car that has a looong brake-way, you should shift slower.
Take my advacie, start racing in slow cars to learn how to manual shift.
Shuffle races online are a very good learning school.
If you want to drive solo, take a slow car, and try to drive it like you would on a 'real road'. Try driving a lap on London or Rome imagining your dad is sitting next to you in the car (and it;s HIS car!)...
Learn in slow cars.
edit: something like this:
http://www.youtube.com/watch?v=qJ3sRKBUqkw&feature=related