You can make Caster Sugar by pulverizing some regular white sugar grains in the blender (a VitaMix with the dry blade does it properly, but it is not really that important and, if in doubt, you can just substitute powdered sugar or regular sugar). You are just making a simple syrup, so it should remain a simple process. However, the Caster Sugar incorporates most quickly, as it is the most fine.
The original recipe called for slitting the cherries, but not cut in half. I used a cherry pitting gadget for one batch and whole cherries for the next. If all you can find is sweet cherries, they will do, but cut the syrup in half or eliminate all together. We prefer the whole cherries, as there is something about the pit which adds to the flavor, perhaps a needed bit of acidity.
Dissolve the sugar in the water in a hot pan, add the cherries. When the sugar dissolves and juice starts to run, transfer the whole lot to your preserving jar and add the vodka. We sterilize the jars in a hot dishwasher, though you don’t need to do this as the alcohol will kill bacteria, but you want the jars as clean as possible.
Store in cool, dark place, shaking weekly or, at least, monthly. Allow them to cure for at least 6 weeks, or 6 months is even better. We have some over 2 years old and they don’t seem to be the worse for wear, though they are probably best in 6-12 months. We keep them in the bottom of the refrigerator. Refrigerate once opened, if any remain, but we believe the best recipe is to simply scoop them all out and serve on their own, on top of a cake, brownies or ice cream, while the vodka is served as an after dinner drink on the side and as part of a syrup glaze. Or just take a long on a picnic, sharing drinks from the jar with friends as an aperitif and then eat the cherries for dessert.