1/2 mg per pound every 8 hours is common wisdom. You will have to cut up the pill as carefully as possible or, if it is water soluble, you could dissolve it in a set amount of water, draw it up into a medication syringe, measure off the appropriate amount and slowly squirt it into your cat's mouth - coming in from the side of the mouth works best. Calibrated oral medication syringes are sold in most pet stores.
It would be best, however, if you can call your vet and confirm the medication and dosage with her. Did she recommend Benadryl? It is pretty commonly prescribed for cats with allergy symptoms, but I would recommend verification, definitely.
I would not give human medicine to my cat if the vet has not recommended it. Some vets will recommend benadryl to cats with allergies. Usually, those allergies are presented by a cat scratching, not a swollen throat and paw. Talk to the vet first. You need not make an apointment. Call the vet and ask him if 1 ml one or twice a day is good for her.
Good luck with your kitty.