What you really want to know is "how to double-clutch." First of all, you should only really down shift if you are not coming to a complete stop. If you are stopping, leave the car in gear as you brake until the rpm's are down to 1500 or so, then press the clutch, put it in neutral, let off the clutch and keep your left foot away from the clutch pedal until it is time to go. Down shifting puts wear and tear on your clutch and synchronizers and it costs a lot more to fix these than to replace brakes.
Before you learn to double-clutch, you can learn to match revs. This just means that when you down shift, you rev the gas a little when the clutch is depressed. If you are down shifting from 5th to 4th, you only rev a little. If you are down shifting from 5th to 2nd, you will need to rev a lot. For example, you can coast down to 35 mph in 5th gear and you'll be turning about 1000 rpm. If you down shift to 2nd, you will need to rev it up to about 4500 rpm or so. But don't try to watch the tachometer when you do this. It's all done by feel and practice. When you match the revs to the car speed, you won't feel any push or pull when you let the clutch out. This means less wear on your clutch and less abuse to the drive train (the car will still feel "tight" after 50,000 miles.)
Now double-clutching: it's exactly as described above but as you shift from 5th to 2nd, you let the clutch out in neutral for a split second as you rev the engine, then press the clutch back in and shift from neutral to 2nd. This is all done in one, smooth motion. When you let the clutch out in neutral, you are connecting the gears in the transmission to the engine so that the gears are revved up too. This way, when you shift to 2nd, the gears speeds are matched and your synchronizers don't have to do much synchronizing and thus don't wear as quickly. In the old days, some cars had no synchros so double-clutching was mandatory. Most cars still don't have synchros for reverse (I think that some BMW's do). That's why it is sometimes hard to get your car into reverse and why you should always be stopped when you shift into reverse. Double-clutching probably won't feel any smoother than matching revs but it will make your synchros last forever.
Also, you will find it much harder to down shift smoothly into 1st or 2nd as compared to 4th. That's because the shorter gear ratios of 1st and 2nd basically amplify the force of the engine.
Good luck!
