Pop your popcorn according to package directions. Add it to a large baking sheet that's been lined with wax paper. Discard any unpopped kernels. 

Melt your white chocolate in a medium bowl in 30-second increments in the microwave (stir in between). I err on the side of microwaving less and letting it sit a bit longer between intervals so I don't scorch it. 

Pour the white chocolate over the popcorn and toss it with the chocolate until it's fully coated. You can use your hands or even salad tongs for this step. 

Spread the popcorn out in an even layer and add the sprinkles. 

Let the chocolate set. I like to speed this process up by putting the baking sheet in the fridge for 15 minutes or so. 

Keep in a sealed container or portion the popcorn up (e.g. in small bags). It'll last for a few days without going stale.