First of all: make sure your cats are healthy!!! It's quite something to travel with pets, especially when they have to travel for many hours, in carrier and also in a plane. You should also make sure that your cats have all the vaccinations required.

Contact the airline you want to fly with in order to get information from them. Every airline has different rules and regulations.

To make your cat more comfortable with the carrier, is leaving it in the livingroom with the door open and a nice cosy blanket in it. Your cat might check it out. You can always talk to your vet about sedation.

When we had big plans in moving to the USA, the best information I got was from the American Embassy. I'm sure your country has the same information available for you.

If possible your cat should travel with you on board. Don't let your cat travel as cargo. The temperature changes drastically there from hot to cold and back again. As travelling is already stressful, the last thing you want is that you cat has to go through that alone in her carrier for many hours.

I'm sure other Pet Talkers have more information for you.