Hi, the first thing is to shop for "4-point harnesses" which are ... collars that have a 2-4" long piece going along the cat's back from the collar to another 'collar' that hooks up behind the front legs (so it's longer to begin with than the neck collar). There should be a ring attached to the piece on the back where you can clip on a leash. Once you have the tummy collar fitted right (not too loose or tight - room for just one finger to slide under it easily) you won't have to undo it again as, once the neck collar's undone, the whole thing just slides off the back of the cat and they learn to step out of it. The whole point being that cats have little skinny necks (compared to dogs) and a neck collar alone is too easy to slip out of, and can break their necks if they get hung up on something, so the harness transfers more control to the tummy part. ANYhow, you acclimatize your cats to wearing it by just putting it on them for a short time in the house, maybe a few times a day for a few days (depending on how much they react, or not) and then at a quiet time of day, bring them to the door, hook on the leash and slowly open the door (don't know if your cats are used to going out or not). Let the CAT go out first and look around (don't drag him out) and then you follow, but let him lead the whole time (unless he's about to get tangled in a shrub!). Never jerk the leash though, just apply enough pressure to stop or lead him away. If he wants to go back inside right away, do it, and take off the whole harness (neck first) quickly, but repeat the whole thing again later on or the next day, til he's used to going out for longer periods at a time. They should wear the harnesses in their carriers, and you just clip on the leads before taking them out - try to get people to move away a bit so they don't go nuts (they can wiggle out of the harnesses under duress, but shouldn't if they're not too upset). I also recommend a pair of gloves (that have been pre-rubbed on the cats at home as a familiar scent) just in case their claws come out in the airport and you need to control them fast.