I taught my youngest golden how to use the bell. When he was by the door I would take his paw and touch the bells, praise and reward. He got it fairly quickly. I left them up(they are still up) and Apollo used them on his own. They are generally now used in case of emergency--ie I gotta go now and you aren't noticing.
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD