What's the most effective way to train a new worker?

Put them near an experienced one. Let them observe and learn from the ones that know the job already.

In the company I work now, we call that work buddy. You’re being assigned a person for your first week who sits close to you and you get to ask them anything you want, work together etc.

Also, as time goes by, write down the most asked questions and issues so people can reference those for simple and recurring questions. That will scale better.