I just mix it in a water bottle by shaking it, on the warmer but not hot side, so that it dissolves quicker, but won't burn the fish when I dump it in.
i just add it directly to the tank. i've been doing it that was for about 5 years now and i've never noticed a problem. actually i've never even considered that it could cause a problem. has anyone had a problem doing it this way? the container that the salt comes in says to add it to the tank, so that's what i do.