Here is an article; send them the link or print it out for them:

https://www.nerdwallet.com/article/taxes/how-to-fill-out-form-w4-guide

Because you have to be very careful on telling people what to do. It's their form and their choice, and you don't want them to come back later and accuse you of being the reason they are underwithheld and penalized.

Work through it with them, if you feel it is a bit beyond them at this time. That helps them learn.