Congrats on your trusty new butter keeper. Our staff swears by them—and here's a helpful (and quick!) video we made to get you started.
Make sure to fill your butter keeper with enough water so that, when it's full of butter, the water touches the surface of the butter. This will also help ensure that the rim of the lid is submerged in the water, creating an airtight seal. We suggest packing the butter fairly tightly at first, so that as you begin to use it, it will still be dense enough to stay in the cup. You'll need to change the water at least once a week, but you could change it as frequently as once a day.
Our best tip: Keep your butter keeper out of direct sunlight, and away from the stovetop, oven, or toaster. All that heat can affect the water seal (read: vacuum) that's keeping your butter fresh. As the weather warms up, you may find it helpful to chill your butter before adding it to the keeper—or you could pop the whole thing in the fridge for a little while to curb any melting.