Hi, I'm Dr Steve. The process is similar to train an outside dog as it is to train ajn indoor dog. The goal is to show her how to distinguish from the correct place to go from undesirable places. With an indoor dog, most people are satisfied if they just go somewhere outside. It is easier for a dog to distinguish indoors from outdoors and they can learn that once they go outside they are allowed to go anywhere.
The problem with training an outdoor dog is that the outdoors is very similar and it is harder for them to know what makes the spot you want them to go different from the rest of the garden. This is what makes it difficult to accomplish what you are trying to do.
The goal is to control where she goes everytime she goes. If she goes in one area when you are not around and then you take her to another place when you are there she isn't understanding the difference. Physically preventing her from going until you are there to take her to the right place is the way to train her. THe best way I have found to do this is just like training an indoor dog. Keeping her in a crate for most of the day and night usually keeps a dog from urinating or defecating where they sleep and then you have control. Take her out of the crate several times throughout the day ( just like if she was an indoor dog) take her straight to the desired location ann keep her on a leash so you can direct her there quickly and once she has gone where you want her to , give her a treat and lots of praise.
If she goes somewhere else and you are right there you can give a stern "NO" and take her to the place. Every time she goes in the right place and is rewarded she learns, but every time she goes somewhere else, she is unlearning. SHe can't logically know what you want so you have to physically control her habits and reward the good ones and either ignore or "punish" with a "NO" her bad habits.
ONce she has gone where you want then she can be alowed to run for a little while with your supervision. Agaain if you leave her alone after going on the right spot and she happens to go somewhere else when you are not right there she is unlearning what she just learned. THis can be tedious and frustrating, but with enough time she should learn what you waant. ANother tip would beb to do something to make the desired place a little different than the rest of the garden. This could be something like putting up a rope fence around a small area where she can go with an opening so she can g through the "Door" like a house and she will understand that the "dor " is where she needs to go. THe rope can even be high enough for her to go under so she doesn't become entangled in the rope. Just something that makes that area look a litle different than the rest of the garden.
I hope this has helped. If it has would you please click the submit button so i may receive credit for my answer. I will check tonight to see if you have more questions, Thanks for using just answer and good luck