There are many ways to acclimate your fish. This is just one I found to be quite effective--zero out of 30+ fish losses reported so far for a beginner fish keeper.
Click to see video on youtube:
It's not solely my idea, but a culmination of knowledge learned from other fish keepers.
Required: airline tubing, bucket, chair
Ask the fish store to add more water in the bag when you first buy the fish.
When you get home, pour the store water and fish into a bucket. Put bucket on chair beside the tank
Tie a knot on the airline tubing--adjust the tightness to control the water flow.
Suck some water through the tubing, let water drip onto holding bucket. It's better to have a dark coloured bucket and lots of shading--fish feel more comfortable that way. I didn't do this.
Water should drip 1-2 drops per second.
After 1 to 2 hours, when the tank water is 2-3 times the initial amount, net the fish into the quarantine or main tank.
I got free stress coat bottle, so I added some into the holding tank. You can do this too, it seems to lower fish stress.
Note: Never add store water into your tank water.
You can put your own variation to this method.