You have to gently pry the clenched jaws open and squirt the fluid slowly and a little at a time against the cheek with a plastic syringe. They don't like it much but when they are that sick they are usually too sick to fight it much. For their own good, you have to force them to take the fluids by squirting it in there. You can buy the plastic medical syringes in the toddler section of Wal-Mart or at any pharmacy.

My cat quit resisting after the first few times and actually cooperated. He knew I was trying to help him and each time he got some fluids he was feeling better.