best way IMO, is to take a SG reading of the lfs water and your water. Also ph of both.
Put water and fish from lfs in a bucket. Tilt at an angle if need be, if there isn't enough water for him to swim in. Then drip acclimate him, or add a cup of your water every 15 minutes until you are within .001 with your sg and around .1 with your ph. No matter how long this takes it is the best way to do it.
Also turn off your lights when you add him and let him sleep undisturbed all night.