I just have one big bell that I got from Hobby Lobby, hanging from a ribbon, and attached to the glass on the door with a suction cup. I started by calling Luna to go out, and making her touch the bell with her nose, saying OUT, after that opening the door. We did that for about 2 days, and after that, there she was, bitting the ribbon, to make the bell ring.
Hope this helps, it worked for us, and is great to have them telling you they need to go out... The dark side??? they NEED to go out to go potty, chase a squirrell, bite the grass, bark to birds, or just check their land... so you will soon find yourself beeing the doorman of your dog
LOL, but it works, so keep trying!