It might be helpful to positively reinforce using a specific area as a toilet.
As an example, when I lived in an apartment, I took my dog out to the same spot (a mulched area) to do her business before we went on a walk. While this was mostly to avoid being a nuisance and killing the grass in the common area, it meant she knew the correct place.
To set this up for you, pick a specific place in your yard (away from your garden or prized flowers, for instance) and take your pup out there, on a lead and on a schedule. This enables her to not get a chance at a wrong answer, like Victoria pointed out. It might also help if you use the same door every time. (Most dogs are bad at generalizing information.)
When she does her business, make a big deal out of it. Pet her, tell her how smart she is, give her a high value morsel. (High value treats are often something stinky, like cheese or cut-up hot dogs, but this can be unique to your dog, too. I had a dog who learned freestyle dancing on frozen green beans alone.)
Soon enough, when she feels the need to toilet, she'll try heading to the most rewarding place to do it - the place you've conditioned her to go. This will probably look like heading to the chosen door on her own or coming to get you, the holder of treats. Reward this, too, when you see it. She's got the right idea, make being right fun and rewarding.
Remember to be gentle when she gets a wrong answer, you don't want her to associate peeing with you being upset, that will further confuse the issue and delay the result you're looking for.