Lexi is litterbox trained. I use a litterbox with a wee-wee pad in it.
I started bringing her outside. I did crate training, and brought her out often. Even though we were basically together in the kitchen most of the day when she was not in her crate (we were doing major renovations, and the kitchen and my bedroom were our only useable rooms), I would occasionally find accidents. Despite being together, I rarely caught her, so she was learning to go outside, but not to NOT go inside.
One morning, there was a very bad thunderstorm. I didn't really feel like going out in the rain, but the lightning was truly dangerous. Since it was first thing in the morning, and Lexi always has to go, I put her right in the litter pan and said my usual "go potty" and "hurry up" to her. I stayed there so she couldn't hop out. She went, and for the first time, despite some of the experts' recommendations against this, I gave her a treat. For the rest of the day, I continued putting her in, as it rained hard all day. I would put her in the pan regularly, as if I was bringing her outside. If she tried to hop out, I'd put her back in. When she went, she got a treat. We continued for several days. I would sometimes put a new pad over one she had peed on already, or if she pooped, I would throw it out, but smear just a little bit on the pad and put a new one on top - just to get the scent in the pan.
After a few days, I heard her go in the pan on her own, and she went! She pretty much kept doing it on her own, and while we don't always see her every time she goes, she still gets a treat most times if I see her, or if I can see that she has gone.
She is now 8 months old, and she was litter pan trained by 4 months. I know we were lucky. On occasion, I will find an accident somewhere, but she is very consistent. If her pad gets too wet, she might go elsewhere, so I try to change it after about 2-3 pees.
Good luck!