CSS3 border image property help

Wondering if anyone has been successful at using a sliced image as a border using css.
Specifically, I’m trying to implement something like what is shown here:
<https://www.w3schools.com/cssref/tryit.asp?filename=trycss3_border-image&gt;
only I want it to look more like this:
<https://www.victoriapatterson.com/about.html&gt;
I have the background image, which I created in illustrator, but I am not sure how to code it into my child theme style.css doc – or whether I should slice it IN illustrator or using CSS (obviously, I really don’t know what I’m doing here)
Any help is GREATLY appreciated. I’m hitting my head against the wall with this one…