To match gears for downshifting, all you need to do is press the clutch, gas it so it revs up, and dump it. When to dump it varies a lot on what gear, what speed, what rpm, ect. It just takes time and practice to know your car.

In other words its the opposite of smooth upshifting. Think about it, when you upshift, you dont gas it. You press down the clutch or you slide it slowly and therefore you let the rpms drop. If you get the right rpm speed, you can dump the clutch as fast as you want, you wont jerk or slow down at all.

If you really want to find out what exact rpm to shift at. Learn to shift without the clutch.

Hypotheticaly speaking...
If you go 15 mph in first gear, and your rpms are 4500.
And at 15 mph in second gear, your rpms are 3000

Then to upshift drive to 4500rpm in first gear, pop it in neutral, wait till it gets to 3000 grand and slide it into 2nd gear.

To downshift from second gear to first gear. Drive to 3000 grand in 2nd gear, pop it in neutral, gas it to 4500rpm then slide it into 1st gear. It'll pop in like nothing, dont force it, if you have to force it then your doing it wrong.

I hope that helps.