I generally get a bucket with 2-3 inches of dechlorinated water at about the same temp as the tank, make sure the pH is the same as the tank, and add a piece of romaine lettuce or some cooked peas, something for them to snack on when they come out.
Open the bag, and just take out any snails that are already pulled into their shells. If they are stuck onto the bag, GENTLY pry them off with your fingers or an old credit card if they're really stuck. You don't want to tear the muscle that attaches them to their shell. When they pull away, put them in the bucket and leave them there for 10 minutes or so, let them adjust. Then transfer them to the tank. Easy.
That way they have a chance to recover in fresh, clean water without fish or other snails to bug them before they go in your tank.