Take it out of the bag and put it in a bucket or the styrofoam box. Begin to slowly drip water from your tank into the bucket/box. When the amount of water doubles or triples in about an hour, scoop the water out down to its original level and keep the drip going until the water in the bucket doubles again. At that point you can put him in your tank.
The water in the bucket should be thrown out, not put back in your tank.
If you don't have some fresh salt water, you need to make some or buy some to make up for the water you loose during acclimation.