Add ice to a lewis bag and hit it with a mallet. You just want to break it; not obliterate it. Hit it a little bit for cracked ice and a bit more for crushed ice feeling for big chunks as you go.
Just because you’re breaking it up, doesn’t mean you shouldn’t use your good clear ice—you’re not an animal.
You can keep your lewis bag full of ice in the freezer if you want to crush enough for a few rounds at a time.