I use about a pack per 3 gallons -- not sure of the exact amount but I think it's roughly 1 tablespoon. It's in that ballpark. All I do to prepare it:
1) Fill a glass with about 2-3 ounces of water.
2) Microwave until it begins to steam hard or boil.
3) Stir in the gelatin. It takes forever to dissolve. Stir stir stir. If you use a clean spoon, the water is hot enough to sanitize your spoon.
4) Allow to cool for a few minutes.
5) Pour it into your cider.
24 to 48 hours later, the cider is crystal clear.
I don't worry about temperature. Just ferment it cold, like in the 50s, and keep it cold the whole time. If you fermented warm, you'll probably get identical results if you keep it warm. I don't think temperature is a huge factor. But I could be wrong.
Does gelatin strip tannins? I don't know. If you're concerned, then add tannin afterwards. I dose mine with about 1/4 to 1/2 the dose that they recommend for white wine. Works great.
Sunlight will not affect your cider at all. However, oxygen can. It's a good idea to keep your fermenters as full as possible. Since cider doesn't have much krausen after the first day or so of fermentation, you can top it up real high to eliminate oxygen intake. If you want. It doesn't matter too much in the short term, but...
If you leave your cider sitting around for several months, it will often turn into vinegar from the oxygen exposure. It's happened to me twice. Not to worry, though -- the vinegar is delicious and if you are resourceful it can be put to all sorts of good uses. Hopefully it doesn't go to vinegar, but if it does, all is not lost.