The best way I've found is to start small, and grow over-time. To explain that further, when a brand-new product marketer starts I will typically walk through step-by-step the messaging process with them at a granular feature level just to get them some repetition practicing. After doing that a number of times, I'll let them take on some of those (very) small releases on their own and will read messaging (i.e. blog content, in-app messaging, etc) ahead of time to make sure they're on the right track.
Once a new product marketer feels comfortable handling the smallest scale feature-releases and messaging, I'll ask them to take on a component of a larger launch (i.e. customer marketing). This gives them a defined chunk of work for a larger launch, without being overwhelming and is still contained. Much like I mentioned above, I'll still read through their copy ahead of time to ensure it's hitting the right tone.
After time of essentially repeating this process throughout multiple aspects of a launch or campaign, then a PMM should be ready to try this on their own. As a manager, it's important to be available for help - but it's also critical to let your team push themselves and rise to a challenge.
Lastly, while the process above has worked very well for me at HubSpot it's always supplemented with core book recommendations like Positioning, and then follow-up discussions around the book and key concepts. I've found that these things combined can be a really effective way to bring someone who may not have any formal product marketing experience onto the team and coach them on how to write great messaging. Good luck!