This depends massively on the abilities of the person you're mentoring.

If they're brand new to programming, they're probably not going to be working on production systems. They might be given tasks to create tools and utilities that might be used by the team, rather than the customer, or they might be given exercises to help them understand programming. The most important thing is to get the right mix of training and programming. Following online tutorials alone is not enough to get someone up to speed - they need to be given software to create without following a guide. They need to have someone they can come to when they're stuck, and they will get stuck. They need frequent code reviews to check they're on the right track. It will take a fair amount of mentoring, and you can't expect them to be able to contribute positively to the team for a while - but when they do get up to speed, they will be extremely familiar with the team's tools, technologies and products.

If they've done some programming before, then maybe they can be let loose on production code. But they will still need frequent detailed code reviews, they will still get stuck and need someone to come to for helpl

When they need help, they do not need someone to show them the answer - that doesn't help them. They need someone to point them in the right direction. To do that, you need to understand why they've failed so far. Which part of the process have they got stuck on. Is it that they haven't understood the problem? They haven't understood the solution? There's a piece of code or a library function they're not familiar with? Work out where they're stuck, and then put them right on the specific thing they're stuck on. Then, ask them what they plan on doing with the new information you've given them, and make sure they know what to do next.