Yep, pretty much like a yeast starter excepting to avoid any aeration or oxygen contact. Pre-acidifying the wort to around 4.5 pH also helps reduce any infection opportunity.
If you haven't tried it yet, the handful of grain inoculation can be an inexpensive way to produce a lacto starter. With pre-acidification and checking the aroma, you can be relatively assured that your starter is suited for use and the only thing you could lose with that method is some wort or extract. The real benefit of this method is that the sour flavors are broader and more complex, unlike the single note sourness that you may get with a commercial lacto strain.