I start by rinsing my grapes under cool water and laying them out on a paper towel. Drying them matters more than people realize, because wet grapes won't let the yogurt stick properly. I give them a good five minutes to air-dry before moving on, which keeps the whole process from turning messy later.
Next, I line a tray with parchment paper and set my bowl of yogurt right beside it. I like keeping everything within arm's reach, since the yogurt can get a little drippy once you start working with it. If you're making a bigger batch, like for a Halloween party, I'd suggest doubling the yogurt so you're not scraping the bowl halfway through.
I hold each grape by the stem end and dip just the bottom half into the yogurt, then set it dipped-side up on the parchment. This creates that rounded "white of the eye" look without fully coating the grape, which honestly looks more realistic than a full dip.
While the yogurt is still wet, I press a small blueberry (or half of a larger one) right into the center of the dipped area. I use a toothpick to nudge it into place if it's being stubborn. This little step is what sells the whole "eyeball" effect, so don't rush it.
Once my tray is full, I pop it into the freezer for about 30 to 45 minutes. This isn't strictly necessary, but I find the yogurt holds its shape much better once it's had time to firm up, especially if the grapes will be sitting out at a party table for a while.