Yes, use distilled instead of Tap water. If your just setting your tank up, fill it with water, keep track of how many gallons you use and then add your salt mix....usually a half a cup/gallon of water. Then add your LiveRock and Sand.
If this is for a pre-existing SW Tank, then add water to a bucket and then the mix and circulate the water and leave it go for a day before adding it to the tank. That way your sure all the salt is dissolved.
I believe ***** carries premixed saltwater....