I did my first castor oil pack recently (I was fasting at the time also).
I don't know if this is a "proper" way to do it, but I just rubbed castor oil on my tummy, covered it with my all cotton t-shirt, the put a very hot, wet washcloth in a baggie and placed it over my t-shirt and under my sweatshirt.
I noticed that I had really bizarre dreams, but slept really well.