Alrighties, allow me to clarify a few thingies!
So! Nutes can come, as far as I'm aware of, in 3 different types:
-Water Solluble (Powders, Crystals, etc)
-Liquid types (should also be put in water)
-Direct/oragnic/however you call it.
The third one is the type with you'll mix it in with your soil, and/or make a tea out of what you have. (Tea as in you boil it up so the nutrient rich.... often poop.... goes into the water, and you wait for it to cool down before giving it.)
Now, for the 20-20-20 question, that's a standard indoor plant nute mix that's widely sold in grow shops or home hardware stores. Although it's not too bad for the plants, it's not an ideal mix.
As for the mixing of raw N, P, and K, that's, as the other man said, a big No-No. Although we often talk about those three elements in particular, your plants will imperatively need trace elements of many other things. Just check a nutrient bottle/bag. They'll usually have things such as Calcium, Magnesium, Potash, etc. These are still VERY important for the dear ones.
You are right, as far as the water goes. Waiting 24h is good to get rid of the great Cl that's in the water, but 72 is best, since then you're absolutely sure that EVERYTHING that could cause trouble is out.
Now, just to help a little with the calculations as far as nutes go...
If the box asks, for example, and this is NOT A REAL RATIO, 10 tablespoons of nutrients per gallon of water, and you want 1/8 of that, you have to make what's an eighth of 10... that's 1.25 tablespoons per gallon. So, if you want to put the nutes for every watering, just aim for an equivalent of 1/8.
Oh! And just for the record, giving your plants nutes for every watering is not such a bad thing as people think. If you give them full force nutes, then yes, that'll overcook them and probably end up with a nute burn. But if you make the ratio right, and you don't get to impatient and greedy, you'll get a plant nice and tall quite fast. Just don't over do it, since that will harm your plant.
AND FINALLY, happy growing!
(Oh, the books aren't much of a help in general. If you look around on the net, and look at different sources for your info, you can usually find a good amount of info for all of your growing needs.)