A family of Microsoft word processing software products for creating web, email, and print documents.
I think I found precisely what you need. This site with the funny name offers a series of PDF-generators to create many kinds of grids. You'll probably want the Squares gallery. There's a lot of customization available, even an option to create PDFs of non-standard sizes (like "1/3 of an A4 sheet").
Once you've download the PDF (it will go into your home folder's Download folder), just drag it over into your Word document, where you can fine-tune its size by dragging from the handles (use corners only to keep the proportions), even select Format Picture > Cropand trim it from all sides (for instance, to eliminate the watermark at the bottom).