Be familiar with your equipment and check it ahead of time. Test your system by turning it off. Make sure your reservoir is big enough, and anti-back siphons work. Always have enough Aquagen packs on hand. Some people acclimate new arrivals for hours, then give them a freshwater dip and keep them in a quarantine tank for weeks.
Some people just open the bags and dump both water and fish in the tank.
In both cases, sometimes the fish are fine and sometimes they are not. As with most topics in this hobby, there is no answer that fits all fish. Depending on the type of fish and how long the fish has been in transit, the acclimation technique may change slightly. First, make sure the fish are healthy and were not caught with harmful drugs. You will have a much easier time acclimating fish that were conditioned before they were packed for you.