Getting the Red Thief
You have to get a double-supercharge which is using two supercharge ramps to increase your speed.

Start where you rescued Isaak (step on his platform) charge down the ramp and jump at the very end while still holding square.

Steer to the right and you will see a path winding around a tree. head up that and it should take you trough a tunnel. Jump as you exit(still holding square) and there should be the top a another supercharge ramp (little pointy logs around it) in front of you.

head down that and jump at the end and you should be heading toward the Red thief's hideout. You may need to glide (let go of Square and hit 'X') at the top of your jump but you should be able to just keep going without gliding.

just don't charge off the other side!

Getting to the Last Dragon (and the yellow thief)
This is almost unanimously considered to be the hardest part of the entire game. To prepare go to the Artisans World and stock up on lives. There are 3 ways to do this that I know of:

1: I will start with the most complicated but probably most likely to succeed. This method requires you to gain speed from 2 Super Charge Ramps, go against the arrows up a third Super Charge Ramp and then jump a regular ramp.

Lay of the Land: Let me take you on a tour of our path first. The starting point is always going to be where you find The Dragon Lyle and I will refer to this as Lyle's Tree. So make your way to Lyle's Tree (Remember you can always replay a Dragon). You will notice there is a Super Charge ramp on Lyle's Tree. Let's take a walk down it to see what we can see. As you walk down you can see a ramp jutting out on the right. This is the final non-super charged ramp that you will jump to reach your goal. (you can jump and glide to this ramp if you wish to inspect it closer) At the end of the Super Charge ramp are two torches, these are found on a few Super Charge ramps. Whenever you use a Super Charge ramp always go between the torches. Look to the left and you should be able to see your goal, the last Dragon on a petrified tree stump along with a couple houses and the yellow thief. To the right you should be able to see the stone tree where you found Isaak (Isaak's Tree). Straight a head is a large circular platform with several pillars on it. Go back up to the top of the Super Charge and Super Jump over to that platform.

There are two ledges sticking out to the right of this platform , the first one is pointing straight at the top of a second Super Charge Ramp and the second sticks out at an angle. This second one is the one you will use when we finally do this. For now use the first one to glide over to the ramp.

This is the ramp that will give you your second speed burst. Walk to the very end of this ramp and look to the right. You will see Isaak's Stone Tree with it's ramp going in the same direction as the one you are on. (you may want to sacrifice a life to glide over to get a closer look.)

Find your way to Isaak's tree. From the top of the Super Charge Ramp facing down the ramp, look to your left and you can see the final non-super charged ramp on Lyle's tree. Walk down the Super Charge ramp at the bottom look to the left and you should see a tree with the last bit of a ramp sticking out from behind it. That is the second Super Charge ramp. Now walk back up and notice that you can see the final ramp as you come up. Notice a little bit of green edge. You are going to have to charge up this ramp (against the arrows) and jump at the green edge over to the final ramp. (again you may want to sacrifice a life to glide over to get a closer look.).

Make'n it Happen: Make you way back to Lyle's Tree and make sure you step on his platform so that when you die you come back here to try again.

Charge down the Super Charge Ramp (do not let go of the Square button until I say so) and hit 'X' at the very end and go straight over to the large circular platform. (do not hold the "X" down or use any turbo functions)

Charge off the second ledge(don't jump) onto the second Super Charge Ramp.

Make sure you get some charge out of this ramp. Jump off the right edge past the torches and make a sharp right over to the ramp on Isaak's Tree. (if you need to aim for the lowest part of the ramp) While in the air try to align Spyro with the ramp.

Charge up this ramp and jump at the green edge over to the final ramp(Lyle's Tree). Again while in the air align Spyro with the final ramp.

Jump at the very end of the ramp and when you reach the top of you jump let go of the square button and hit X to start gliding. You should glide right over to the last Dragon.

If it looks like your not going to make it aim for the left. There is whirlwind and you may get caught in the very bottom of it. If you just let it go you'll end up back at Lyle's Tree, but when the whirlwind let's you go you can turn around and try to land (hit triangle to drop) on the last Dragon's tree.

2: Here's an alternate way I found at SpyroHints.com. Charge down the first ramp but jump to the left toward the last Dragon's tree and glide at the top. Head for the whirlwind and you might get caught in it. when the whirlwind let's you go you can turn around and try to land (hit triangle to drop).

3: Here's another way I found at SpyroHints.com. Charge down the first ramp but jump to the right. Hold the controller to the right and try to line up with the final ramp. Jump at the very end of the ramp and when you reach the top of you jump let go of the square button and hit X to start gliding. You should glide right over to the last Dragon.

If it looks like your not going to make it aim for the left. There is whirlwind and you may get caught in the very bottom of it. If you just let it go you'll end up back at Lyle's Tree, but when the whirlwind let's you go you can turn around and try to land (hit triangle to drop) on the last Dragon's tree.

Invincible Thief - You can try to cut off the red thief by first getting Lyle and super charging over to Isaak's Tree (see above) but when you get to him he's INVINCIBLE!. Flaming and charging won't hurt him unless he's at his hideout.