you should let the bag sit for 30 minutes to adjust temperature, then add a half cup of water ever 15-20 minutes after till the bag is full. Pour out half the bag, and repeat till full again. Then you want to pour the bag out into a large soft net to drain all the water and plop your new fish in. It is good practice never to add unknown water to your tank, as they might treat with copper or other medication that could kill things in your tank.
You should consider a quarantine tank or the drip acclimation method, both are explained in detail here with some quick searching. They reduce stress and if you quarantine and catch a problem fish before he comes in, will save yourself a huge headache.
Quarantine for saltwater fish