If I'm lazy, I use a surgeon's loop to just connect it, however, I've found much better results with tying onto the bend of each fly up with a cinch knot. I'm pretty sure the surgeon's loop adds some pretty wicked spin into it, but when I'm using a dry as an indicator, I'm not so inclined to care.
As to how far, it depends on what I'm doing. I most commonly use a big bushy dry as an indicator, and then hang a nymph or a wet and a nymph under it. The wet is usually 12" or so back, and the nymph goes down to as far as I think it needs to.
If I'm using a second dry fly, I'll more likely keep it shorter, 12" or so. I want to have an idea where I need to focus to see where the second dry is, although admittedly I haven't tried this method much.
All wets? About 12-18" between each fly on the cast.
I've never done a streamer/wet combo, but I know alot of people do.
Limit your false casting to as little as neccessary, and they say never roll cast (whatever, I still do, but it tends to end poorly after I've done it too much).
Be warned, tangles will become a new level of awful like you've never seen before, I routinely give up and cut the whole mess off. I also develop a bird's nest if I ever have the misfortune to tap my rod on the cast with one of the flies.
And losing 3 at a shot stings.
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