Bell training is great because it teaches them to give you a signal when they want to go outside. A lot of dogs learn quickly that the preferred spot to potty is outside - especially if they learn potty outside = treat
The problem is often that they don't know how to communicate that they need to go out. Loki would gently scratch at the front door and whimper and i wouldn't hear him. Sam lays on the mat - but if I'm upstairs how do I know? Some dogs walk by the door, start to circle, bark (but you don't know why) or other signals they basically "make up" The bell is a great way to help them choose a signal you can both understand.
You just put a bell by the door (either a register bell on the floor, or a bell on a ribbon hanging next to the door - two products out there are Tell Bells and Poochie Bells) Every time you go out, take their paw and ring the bell, act like they did it
and praise them, and then go directly outside. After they get it, start encouraging them to ring it without help. If they stand by the door say "ring your bell" and encourage them to do it. Again praise them and go right outside. If they do it on their own, even if it's just to try it out, go right outside. ANY time they ring it, go right outside. This makes them associate the bell with going out! Don't worry, once they are trained you will learn if they are ringing it for real or not. Mine don't ring it just for "fun" but they do ring it when they want something like more water. We just figure out what they need and then we all go outside anyway.
They also went from having no signal to ring bell...wait... ring ring ring... wait... lay on the mat and be cute...wait... whine, bark, run in circles, come get me and bark at me, etc. when they want to go outside. When they were puppies it was more like ring..wait..pee on floor if no one comes
So I would just have to drop what I was doing to let them out (I don't now, they can wait 5 minutes as long as I let them know I heard them) I'd rather drop what I'm doing to let them out than to clean up an accident!
I hope that helps and good luck!
OH, I almost forgot. The very most important aspect of Yorkie house training is going outside with them to give them the treat as soon as they go (AND make sure they actually go and are not being sneaky!) Otherwise if you just give them a treat when they come back in they have no idea why they are getting the cookie. Do that for the first month or two until you get a good routine and it will help tremendously. So many people just let them out in the backyard to explore and then they come back in and pee
You have to go out with them and make sure they are empty. I still go outside with Sam after dinner to make sure she poops after dinner because otherwise she gets distracted by birds, squirrels, leaves, etc. Now I mostly just stand on the porch and just watch them, but I remember many days when training them that I spent walking back and forth on the driveway waiting for them to go!
Feeding on a schedule helps too. It helps you predict when they need to poop.
Do 2 or 3 meals a day and you'll get 2-3 poops a day, generally right before or after a meal.