This is another subject where people go really over the top and worry too much, everyone chill out and stop panicing, your fish made the journey of thousands of miles in terrible conditions to end up in your lfs so theyre not just going to roll over and die because of the short trip home.
If your lfs is in the same area as you live you do not need to acclimatise the fish to your tapwater, the lfs should have already done that before putting them on sale and if they havent the dont use that shop (unless of course you have knowingly bought a species with specific requirements in which case you should already know what you are doing). On ariving home with your fish float the bag in the tank for around the same length of time as the fish has been bagged to allow the temperatures to synchronise and then release them, never allow any of the water into your tank.
If the fish have come from further afield and are in a different water to yours then you need to acclimatise them slowly, the best method for this is the drip method. Place the bag containing the fish into a small empty spare tank or waterproof box, i use a poly box for this, open the bag and fold down the sides so the the top remains open and prop the bag up so it cannot fall over. Next take a length of airline that is long enough to go from half way in your tank to the floor, place one end in the tank and in the other tie a loose knot. Give a good suck on the knotted end to start a syphon (making sure not to get a mouthful of tankwater, this may take practice

) and place the knotted end into the open bag containing the fish, then adjust the knot until the water comes out of the airline one drop at a time. Allow the bag to overfill and start to fill the tank/box, when there is enough water to cover the fish you can release the fish into the tank/box. After around two hours the fish will be acclimatised to the new water and tank temperature and can be released into the tank that will house it.