With a bowl or tub or small tank filled with feshwater, dip the fish in.
The water of course should be clean filtered water like RO water.
It is also best and a little less stressful to the fish if the temp is the same as the saltwater from which the fish is removed and also the same PH is best.
The amount of time I would leave the fish in the freshwater would depend on how well the fish handles the stress. I would not go more then 5 - 10 minutes though.