You hang a bell on a ribbon or use a register bell (the dog-themed products on the market are called the Tell Bell and Poochie-Bells, but use your imagination if you are a DIY-type person...) Anyway, when you take the dog outside, take their paw and ring the bell. Act all happy, like they did it, and go outside! Repeat 10 or 20 or 100 times until the dog does it on their own. When they do, praise them and IMMEDIATELY go outside. Bell=outside, even if they are just messing around. Eventually it clicks. Loki took about 3 days to get it. Sammy took around 6 weeks, even with Loki using the bell. Then one day she got and has used it since. It teaches them to ask to go out! They generally understand they need to pee outside (assuming you are taking treats out with you, watching them and praising/treating them as soon as they go) but they don't know how to *get* outside unless you take them.
Start when they are puppies and they have no idea when they need to go out (their bodies don't start giving them any warning until they are like 4 months old, and still very little until 6-8 months depending on the dog) You still need to go out on a schedule, but practice the bell! Once they start to get a little warning they will know just how to ask! You can also ask them to ring the bell before you go out. Mine give me a look, like "duh you are already here!" but they do it.
My issue was that Loki was trained but he didnt know how to ask to go out. I would be upstairs and think he was napping somewhere in the house and come downstairs to a puddle in front of the door. Unless I was sitting right there and saw him standing there, i had no idea. So, once we got the bell he rang it and I came running! I felt like a butler, but overnight we had no more accidents
(Again, he was already like a year old and trained, but it made all the difference!)