My 2 cents (and my first post here) - I've raced cars with Skip Barber for about 8 years, so have a decent amount of shifting experience. Some important points about downshifting:

1) You need to be braking hard for it to work well. If not then your brake pedal is too far away from the gas pedal to blip the engine enough.

2) Practice blipping the throttle when you are at a stop. With the car in neutral, press the brake hard until it is even with the gas pedal. The left side of your foot should be on the brake pedal, and the right side should be over the gas pedal. Make sure this is the case. Then just roll your foot to the right a bit to push down on thegas pedal. You want the engine to rev up about 2000-3000 revs. Try to keep the same pressure on the brake as you do this.

3) Once the above step is mastered the sequence for downshifting is:
a) Brake
b) Put clutch in
c) Put car in neutral (while still braking)
d) Release the clutch
e) Blip throttle as explained above
f) put clutch in
g) go into lower gear
h) release clutch
Note that these steps all happen in about 1/2 a second. When getting it right, do it in a straight line, because if you get it wrong in a corner you could spin. Also if you mess up the blip, either little or too much, you can always let the clutch out slower to compensate as this is a road car.

Good luck!