Note that the temperature drops in the deep fryer when the onions are added. So be careful not to add too many at once. It is advisable to make them in two batches.
If you don't have a deep fryer, you can cook them in a pot with oil. If you don't have a thermometer, you can use the rule of thumb by placing a small wooden stick or onion ring in the oil before starting. If it sizzles around it, then it's ready to fry.