Just took a moment to read heel and toe somewhere. How do you know how much revs are enough for the gear that you're going to shift into? My instructor taught me, shift to second when the rev meter hits two, shift to third when the rev meter hits three.
So when downshifting, should I apply the same principles or is there any other way?
I don't really like these kinds of "hard rules". While they work for most cars (with typical gear ratios to power bands), they don't always work perfectly, and they require a tachometer (not all cars, not even all manuals, have them!). You also have to look at the tach.
What I tend to tell new drivers (or those new to manuals) is to get a feel for the car and learn to shift by the inputs its giving you. At first, it's something like this - if you give more throttle input and the car doesn't accelerate as expected, try a different gear. If the car isn't going and engine noises are really loud and/or high pitched, shift up (say, from 3rd to 4th). If the car isn't going and engine noises are quiet and/or lower pitched, shift down (say, from 3rd to 2nd). As you practice, you'll get used to how the car should respond to the throttle and how it should sound, and you'll get a more refined feel for when to shift.
Don't worry too much about downshifting as you slow down until you grow accustomed to "feeling" the car in this way. Once you are used to it, you can downshift when you know you'd have to to accelerate, if you want. I don't always do this on the street, though. On the street, I will downshift to the low gear while slowing shortly before I'll need it, typically, but other than that, I won't bother. Some people will tell you to downshift so that you stay in a gear that will allow you to accelerate should the cars behind you not slow. While this can help avoid a collision, it's something I only worry about when there are unobservant cars behind me. If you're not comfortable downshifting yet, don't worry about it yet.
For rev-matching: No need to worry about this until you're comfortable with shift points and want to start working in more advanced downshifting techniques.. But you've asked questions about how to bring the revs up (increase rpm) effectively. I blip the throttle (mash down on the throttle then release) and let the revs climb to near redline (or well above the revs they'll be at in the lower gear). Then, as the revs decrease, I engage the new gear. It's very difficult to have the engine revs exactly match the RPM in the new gear, and if you're imprecise, a little high and dropping causes less upset to the car than a little low (and thus needing to climb). Ways to practice this and more advanced techniques can come later, once you're ready to play with them.
