Your going to get a lot of different thoughts on this subject.
First off you float the bag in the tank with lights off preferably, for about 15 minutes to let the temp change to the one in the tank.
Then you open up the bag, and pour in a cup or half a cup depending on how much water is in the bag. Then repeat this every 15 minutes. After the bag or you've done this about 4 - 5 times.
Pour half the water in the bag out and repeat again 4 - 5 times.
Then its time to put the fish or invert or corals in the tank.