Every time you take your puppy out, encourage him to ring the bells. Show him the bells every time he goes out the door. Use a command like "Bells!" or "Ring!" to help him associate the bells with being able to go outside and potty. Gently take his paw and touch it to the bell. Shake his paw to make the noise and say your phrase or word. Immediately open the door upon ringing the bell. Make sure the bells are rung, even if you are doing the ringing.