Walk your dog out on a leash EVERY time and use a specific phrase (like 'Go potty). This means no matter what the weather, or what else you moght have to do, you go out with the dog to the place you want them to go. After many trips, when you think they've got the idea, remove the leash when you get to the spot. If they 'go' where they should, praise like crazy. If they try to 'go' somewhere else, put the leash back on and lead them to the right spot.

One day you'll realize that they will always go in the right spot.

Patience and consistency!