One jar of cherries in light syrup. They're sometimes called Morello cherries or sour cherries (though they're anything but). Essentially, they're just preserved, pitted cherries in sugar and water. The size of the jar tends to vary with the brand.
Two to three cups of amber or dark rum (anything but white)
Pour out and put aside half the contents of the cherries and their juice. Into your half-full jar (hey, I'm an optimist) pour enough rum to fill the jar 3/4 of the way to the top. Shake well.
Move the jar to the refrigerator and let sit for at least eight hours, and up to a month. The flavors mellow and develop more fully the longer you let it sit.