Set aside roughly ½ cup cider for later. You can just leave it in the gallon or container it comes in.
Fill ice cube trays with 5½ cups of cider. This may depend on the size of your ice cube trays and cavities, but 1¾ cup is roughly the capacity of one ice cube tray for me, so 5½ cups fits roughly in 2 trays.
Store in freezer and freeze until solid, best overnight. (You can also do this in batches, just remove the solid cubes to a freezer-safe container and repeat.)
Add the frozen cider cubes to a blender. Add ½ cup reserved cider and blend on medium until appropriate slush texture is achieved. I like it liquid enough that the cider liquid will pool if you make a cavity in the slush, but frozen enough it’ll keep the spoon’s shape. If too thick or having trouble blending, add water 2 Tbsp at a time until desired consistency is reached.