Put your florets into the food processor. If it's a small head, you may be able to do this all at once. If not, don't be afraid to do it in batches. Sometimes you'll get a rogue floret that refuses to get chopped. Don't keep going at the batch until you get it. When the batch is the size of rice grains, remove it. Add any un-chopped florets to the next batch. (Some people claim to do this on a box grater. I never have and it doesn't sound like much fun.)
Heat your fat in a pan over a medium flame. You can do this in a skillet or a pot. Remember, this is basic. You don't have to get too picky about things.
Add your onions. They'll sizzle a bit. but they shouldn't screech. Keep cooking and stirring until they're soft and semi-translucent. This is the stage where you will add anything else the you want to cook with the rice — garlic, chopped peppers, celery, carrots, pine nuts, raisins, etc.
Cover the pan or skillet and let cook for 5-10 minutes. If you want to keep your eye on it, you can also leave the top open and stir the rice to move the cooler "rice" on top to the heat on the bottom. The cauliflower should retain some bite. Don't let it go to mush — that's mashed cauliflower.