There’s a lot more detail in the New York Times article, “Groundskeepers Display Artistry on the Diamond”.
I was immediately reminded of Crop Circles. There’s a bunch of geometry there, too, though it’s more about circles than stripes. Like this one:
Or this one:
Or this little guy (by Perfectblue97) which looks simpler but might have been just as hard to make:
There are some more designs at CoolMath4Kids, and more about how to physically make the circles on WikiHow. But if what you want is a step by step description of particular designs, your best bet is to head over to Zef Damen’s page, which has lots and lots of designs (updated frequently, too!). I think I’m totally going to assign this as a creative assignment in Geometry when we’re using a straightedge and compass.
All of which is to say that groundskeepers aren’t the only ones who get to make neat designs.