Let’s take a moment to talk generally about trademark and merchandise. We’ll use NaNoWriMo as an example since it’s a trademarked term. And, evidently, some Wrimos like to court disaster.
If you have a personal notebook and you write NaNoWriMo on the cover, you’re not in violation of trademark laws. That’s personal use, and it would no more be infringement than writing Walmart on your shopping list.
If your sister is just starting NaNo, and you give her a similar notebook, things get a little more sketchy. But only just. Since both notebooks are for personal use and since it’s a one-off, it’s most likely not infringement. In any event, nobody will care.
If you have a private NaNo group and you make similar notebooks that you give away for free to group members who win contests and such, then you could be infringing on NaNoWriMo’s trademark. It would become a matter of how aggressively the mark owner defends their trademark.
If you give away the notebooks–or even the stencil people can use to make their own–freely and to all comers, you are almost definitely infringing on the trademark. No matter your good intentions, you can’t do that.
If you are selling the notebooks or the stencils, expect a Cease and Desist at the very least. At that point, you’re essentially lawyer bait.
In short, you need permission or a license to make any merchandise bearing the trademark that isn’t for your own personal use. That includes tee shirts, pens, coffee mugs, anything. Even flyers or webpages used to announce meet-ups.
But I’m helping spread the word about NaNo is not an effective argument. Remember that trademarks have to be defended or the owner could lose their mark.
Note: that does not apply to using trademarked names in your novel. Doing that is perfectly fine.