Break the cauliflower into small florets, place in the food processor, chop until the size of the rice. I recommend not to crowd the cauliflower in the mixer, it's better to do this in 2-3 rounds, so that it doesn't turn into porridge.
If you don't have a food processor, you can use a potato peeler or a classic grater. But it will take a bit longer and it won't come out evenly.