I did this with Roxie and am starting with Falken now. I started the day I brought him home, at about 10-1/2 weeks. I have the bells hanging from the back slider door, where they go out to do their business. I put the pup in a sit to put on his leash, say "let's go potty" and then ring the bells (which are at a height right at Falken's nose). It's been about 2 weeks so now I'm starting to put Falken's nose to the bells to ring them so he gets the idea. The key is consistency, every time they go out to potty ring that bell. (I admit it's taken me a little time to remember to ring the bell myself) And they go out to potty and right back inside, no playing after ringing the bell. As soon as they figure out they go out when they ring the bell they will start ringing it just to go out and play !! Roxie got the hang of it very quickly, within a couple weeks. Falken is taking to it slowly, he's rung the bell 2 or 3 times on his own but not sure if it was on purpose or not LOL!
Good luck!