Children's Benadryl liquid is good. You give about 3cc's (mls) every 4 hours or so. I was advised that here and also by my vet.
I believe thats what I was told also. You can probably just call your vet & confirm the amount for your dog. They may even give you the syringe that you need to measure out that amount. I know Children's Liquid is what they told me.