I have taken away my yorkies pee pads

and am trying to just get her to go outside only (unless she is in her xpen while I'm gone). I am taking her out every half hour right now but she is only 5 mo and still has accidents in the house. In the evenings sometimes she'll go every 15 min!
I live on the 3rd floor of an apt (with no elevator) and I'd like to train my yorkie to ring a bell to let me know she needs to go out.
How did you train your yorkies to use the bells?