Interesting article from the American Kennel Club
Teaching your puppy To GO Outside
Teaching your puppy to ring a bell to go outside is a great trick and very helpful for house-training a puppy. This method may be preferred over having your puppy bark at the door or just standing at the door when you may not realize he is there waiting for you to take him out.
The first step is to hold the bell close to your puppy's nose. Once the puppy touches the bell, say “yes!” and give the puppy a treat. If needed, you can encourage the puppy to touch the bell by holding a treat behind it or dabbing some peanut butter on the bell. Repeat until your puppy readily touches the bell with his nose, always giving lots of praise and a treat. When he confidently touches the bell as soon as you present it, add a word such as “touch” or “bell.” Start to hold the bell a little farther away, so the puppy has to make a few steps to touch it.
Once your puppy has learned to touch the bell on cue, hang the bell from the doorknob on the door where you would like for your puppy to go out. Encourage your puppy to touch the bell, and as soon as he does, say “yes!” and treat. Repeat until your puppy readily touches the bell when you give him the cue. Reward him and give him very enthusiastic praise!
Step three is teaching your puppy to ring the bell at the right time. Approach the door with your puppy, point to the bell, and say your cue, “touch” or “bell.” The moment he touches the bell, say “yes!,” open the door, and take him outside. With repetition, your puppy will learn that when he touches the bell, you will open the door.
If your puppy begins to ring the bell just to go outside to play, you need to teach him that ringing the bell is only about potty time. When he rings the bell, clip on his leash, and take him to the place where you want him to eliminate. Give him a few minutes, and if he does his business, praise and give him a treat. If he does not, take him right back inside.
Remember to make your puppy's training a fun game with much praise and tiny treats. Everyone in the household should follow the same training method to avoid confusing the puppy. Know that your puppy has a short attention span, so have brief training sessions several times a day. When you know he has caught on, you can give him a jackpot of several tiny treats, one right after another.