Where eye goggles, gloves and a protective mask. All cheaply atainable from the hardware store. Use only in a well ventilated room or outdoors.
Prepare two containers. One for the dip and one as a rinse. Using a third (plastic) container with holes drilled in to easily transfer the fish will help. Have that container in place before adding the fish for easy removal/transfer. The rinse should be newly made SW at the same parameters as the QT and the dip. Both should be maintained at the same temp but be very sure that stays near 78° and no higher than 80°. Formalin consumes O2 very quickly and will become toxic so temp needs to remain lower. Aerate the dip ½hr before and throughout the process while the fish is in the dip. Add the Formalin at a rate of 20 drops per gallon and wait until the cloudiness clears. Do a final check on temp and pH. Add the fish and place something permiable over to prevent jumping.
Watch the fish very carefully during the process. At any time if the fish starts thrashing, remove it. Playing dead would be normal though. Keep ambient room light low if possible. The entire process will last about 45 min. Every 5ish minutes or so be sure you check the activity of the fish. Try to maintain the dip for a minimum of 30 but preferably 45 min. Once the time is up, transfer the fish to the "rinse" container for a minute or two and then ease into the QT keep the light off. If all goes well, repeat in 3 days and then again three days after that.