We adopted Zoe at around 13 months and she was damn near feral when we got her. I think she escaped from a puppy mill and then lived on the streets until animal control picked her up.

Anyway, she wasn't potty trained in any way, except to think rugs/carpeting were pee pads. (Puppy mill trait.) It took me a year and a half to get her trained but that was because I kept trying different things with her and wasn't very consistent. What finally worked was this:
  • Figure out when and how often she would pee.
  • Take her outside (leashed) consistently, ahead of her pee times. I started taking her out every 30-40 minutes.
  • Do not let her go back inside until she does something. Then praise, praise, praise. You can give a treat if you like.
  • It helped to get her used to peeing outside by taking her out to a place where other dogs went. Then she naturally wanted to mark the area.
  • Once she got the hang of peeing outside, I increased the time between trips. This wasn't even much of a gradual increase. One day, something clicked with her and she would rather pee outside than inside.
Obviously, it's hard to start a routine like this if you work. I started training Zoe the night before a 3-day weekend. At first she would have accidents if I was delayed at work by an hour. Now she can hold it almost no matter how long I'm at work.

The thing about puppies is that they really can't hold it very long. They need to go when they need to go, often up until about a year. And small dogs have a worse time with this than big dogs.

I hope this helps. It's pretty much what everyone says to do to potty train a puppy. The difference for me was the leash aspect. You can't just put her outside in the yard at regular times--you need to take her each time, preferably to the same spot.