First of all make sure kitty has a very safe place outside to go potty, free from predators far away from busy or not so busy streets and alleys and out of sight from any possible cat hater.
Shift your cat's litter box a few feet away from its normal spot, in the direction of the door you would like the feline to use. Show your cat where the box is and let her/him use
Take the box slightly further away from the new spot and nearer the direction of the door. Maybe move it into a different room and or new part of the house and again show the cat where you placed it. Let kitty to eliminate a few times in this new area.
Place the little box in front of the door you would like him/her to use to get outside. This will help your feline become acquainted with the door's presence while he/she eliminates. Take your cat to the box and let kitty use it a few times.
Place the litter box just outside the door. Take your cat outside and gently put him/her in the box to show kitty where the litter box now is. It may take your feline a few times to get used to going outside to situate the box, so let kitty use the box a few extra times before moving the box again.
Take away the litter box altogether once the cat is used to going outside to eliminate. Observe the cat, and when he/she starts showing behaviors that display the need to eliminate---such as digging or pawing at the ground---move kitty to the designated elimination spot. Cats classically like sand or freshly dug soil; consequently, consider delegating a spot that contains these materials. While your cat may use your area you choose, your feline will probably find other spots he/she prefers as well.