Crushed ice is a must for this recipe. It helps to dilute the drink (which is quite strong). You can use shaved ice, if you have an ice shaver, but if not you can make crushed ice relatively easily.
Place cubes of ice in a (clean) kitchen towel and wrap it up so you have a handle with the edges. Hit the towel repeatedly against a hard worksurface until the ice has smashed up into smaller pieces.