If you click on one of the J-cards on the site you linked, you get to a PDF that tells you the exact size of you card after it’s been cut. With that, you can create an area of that exact same size (tables, text boxes, tab stops – knock yourself out) and write your copy text in there. Just be sure to extend any artwork beyond that line (so that you get no ugly white borders after cutting it down to size) and keep in mind the margins of your printer. For the reverse side, just mirror the area of the other side (assuming you centred it, so it’s always the same distance from both sides ) and duplex-print.
It might be a bit of a pain do do it in Word but should be doable 🤞
Best of luck ^^