Yvette Van Veen of Awesome Dogs wrote the following:
Step 1 - Show the dog the bell
Show the dog the bell and gently ring it. Let the dog become familiar with the noise it makes. Do not skip this step. Some dogs startle if they touch the bell and are not expecting it to ring. If you have a sensitive dog, muffle the sound and gradually let it ring louder.
Repeat until the dog is comfortable and relaxed with the noise.
Step 2 - Teach the dog the bell means treats
Put the bell in your pocket or other location that is out of sight. Periodically bring it out of hiding. Immediately give your dog several small treats. Hide the bell. Stop feeding treats and do not praise, coddle or otherwise give unnecessary attention after the bell has been put away.
Repeat until the dog happily startles when it sees the bell brought out of hiding.
Step 3 - Let the dog get impatient and touch the bell
Bring the bell out of hiding as before but withhold the treat. Hold the bell close to the dog's nose. (About 10 cm. away) If you did enough repetitions of step 2, the dog should be confused and frustrated that you are not feeding any treats. As it gets impatient, it may bark. Ignore that. If your dog is a jumper, try doing the exercise sitting in a chair. Eventually, your dog will brush its nose against the bell in an effort to "point it out". It thinks it should be getting treats because the bell is present. When it pushes its nose against the bell, say “Yes!” and give a treat. Always feed a treat if say the word, "Yes."
Repeat until the dog is touching the bell as soon as you bring it out of hiding.
Step 4 - Hang the bell on the door and keep your hand on the bell's strap
Repeat step 3, but hang the bell on the door you intend to use. Keep your hand on the strap.
Step 5 - Gradually start taking your hand away
Hang the bell on the door. Move your hand 1 - 2 cms away from the bell. Click and treat each time your dog touches the bell. Gradually move your hand away a centimeter at a time. There will come a point where your dog may touch your hand rather than the bell. Ignore that. Give your dog a little time. Chances are it will try to touch the bell in a moment or two. If it does not, then temporarily move your hand closer to the bell. Review easier steps and gradually move your hand away when the dog is ready.
Repeat until your hand is completely away from the door and the bell.
Step 6 - Start walking away from the door
Take one step away from the door. Continue letting the dog ring the bell. Each time the dog touches the bell, say “Yes!” and treat. Gradually move further and further away from the door until you are approximately 5 meters away.
Repeat until your dog is quickly and consistently ringing the bell when you put it up.
Step 7 - Start using it before going outside
Before taking your dog outside, hang the bell on the door. Let the dog ring the bell once. Say “Yes!” and treat. Then take it outside - on leash - to go to the bathroom. Do not use the bell for playtime or it will become a nuisance. Stop rewarding the dog for ringing the bell when you are not planning on going outside. It's time the dog starts learning that the bell means potty break.
Repeat until the dog automatically rings the bell prior to going outside.
Step 8 - Hang the bell on the door and let the dog figure it out
Continue rewarding your dog for ringing the bell prior to going outside. But also be prepared for a moment of genius. At some point, you will hear the bell ring and you must be ready to reward your dog with a treat and with a bathroom break. However, do not give your dog free play. Reserve the bell for bathroom breaks only. If your dog does start to use the bell excessively, then take it off the door when you return to the house for 2 hours. Put it back up after a reasonable amount of time has passed. Make sure your dog understands the rules - the bell is for bathroom breaks only.