First, decide on the area, and mark it off in a very visible way. She constructed a 3-sided fence. I think one side was actually the existing fence around the back yard, and the other two sides were this white picket-fence type arrangement.
Then put down mulch or something like that.
THEN - this is the important part - EVERY SINGLE TIME you take the dog out to do its business, take it to that area and that area ONLY. When the dog does its thing, praise and reward. Give a name to it if you are good with timing, so that the second the dog raises its leg, say softly "Go potty!" (or whatever word you use) so he ties the command in with the action.
(If you don't use your voice, give a hand signal.) You can also try using a clicker, so that you mark the desired behavior with a "click." (Google "clicker training" if you don't know what I'm talking about.)
BE PATIENT. It takes repetition and being extraordinarily consistent, which means EVERY. SINGLE. TIME., come rain, sleet, snow or sun, you take that dog to the one place you want it to use, and you are patient until he does it.
My dogs will consistently "go potty" on a gravel area under the house here, which is helpful when we get one of those notorious Outer Banks northeasters blowing through, with a lot of rain.
They will also do their business on walks, since we are not strict about the "one and only one" place. I just wanted to make sure that they know what the gravel area is for, and we use that often enough to reinforce it.
In my case, I also have a backyard agility set up, and they are NOT allowed to do anything on that grass. If they ever compete (well, it would be Casey competing), he would be disqualified if he ever has an accident in the trial area, so I want him to know there are some places that are ok while the agility area is definitely NOT OK for bathroom activities.