I like to start bell training with Touch or Target Training. I put a treat in my hand and ask the dog to "touch" with his nose. When he touches my closed fist with his nose, I release the treat (click/reward). Then I move to other objects, reinforcing the "touch" command.
Then work with the bell and use it as described. When the dog touches the bell, the click (or vocal marker "Yes") lets the dog know he's correct and then OPEN THE DOOR, which is to teach the dog that touching the bell results in the door being opened.
Just my way of teach it, step by step. I find with younger dogs, who are barely out of basic housetraining or still being housetrained, it works better if we take it slowly and in very small steps.
Of course, that's not to say some dogs just get it almost immediately. Maggie got it almost instantly. Showed her twice and that was it. Susie took a bit longer and she still has to be ignored once in a while because she doesn't "remember" she's supposed to ring the bell to go out/come in. Every dog is different, so respect those differences and set your dog up to succeed. Don't get frustrated if he's slow to get it. He will with patience, love and persistent consistency.
Good luck.