The basic idea of heel and toeing is to match the engine revs with the wheel speed whilst downchanging and braking, so effectively your right foot has to do 2 things at once. You know when you change down through the gears and you let the clutch out, the front end of the car dips and it will usually pull to one side, well heel and toeing eliminates this, which helps braking stability (esp. in the wet because it eliminates the chance of wheels locking when you let the clutch out) and saves your clutch!
The way to do it is first of all get used to braking with only half of your right foot on the brake pedal, this is really 99% of the work, finding a comfy position for your foot so it can brake and blip the throttle.
Next step is to get used to maintaining a constant pressure on the brake with one half of your foot, and rolling the other half round to catch the throttle. I found that practicing this with the car stationary and the engine running (so you have correct brake pedal travel) was good, as it saves any scary foot slipping off the brake pedal moments!
This is the order things go:
2- Clutch in.
3- Shift gear and blip throttle whilst still braking.
4- Clutch out.
This all has to happen within a second in the end!
Now try it on the road, at first try 5th to 4th, as this means you are further from the corner and can concentrate on it more! When you feel comfortable with that, try 4th to 3rd, and so on until you can go through the whole gearbox, although you will find this will take a few weeks to get right!.
You need to make sure that your foot doesnt come off the brake and you give the engine the correct amount of revs when you blip the throttle. So in effect you want to ensure the car doesnt jerk forward (too many revs) or dip further (too little revs). It will take you a while to 'know your engine', with respect to its gear ratios and how much throttle you need to apply, this comes with time!
It needs patience because at first you will have to make braking distances longer to give you more time to heel and toe, but you will know when you have it right, as it will become second nature and you wont think about it anymore (I do it literally everwhere now, town, turning into my drive etc, and im never even going fast!).
Once you've cracked it you will feel the difference straight off, and id predict on a track it is worth around a second a lap once you have it perfect.
Just as a note IMO 205's have perfect pedal spacing/pedal height to heel and toe!
Phew! I think thats it! If you have any questions, PM me, as this is one of those things thats quite hard to explain on paper!