You should not get a tattoo without being 100% sure about the image. How can you say that you are getting a tattoo tomorrow, and not know what you are getting? I think that you should cancel the appointment, and wait until you really know what you are doing! This is permanent, and you do not want to get something and a week later wish that you had gotten something else!
Never get a tattoo when you are unsure of the image.
You have a beginning of an idea, but it is hardly fleshed out. A cancer ribbon is all well and good, but it is not as personal as it could be, and there is many ways of doing it anyway. This is about your grandmother, and you should make it so. It should have something to do with her, and your relationship to her. It should not just be about cancer.
It should make you think of her, and her life. She was so much more than what caused her demise.
I think that you should wait, and work on this and personalize it.
There is no hurry.
She is your grandmother. She isn't just the cancer.
Here's a good article about how to get a great tattoo without regret: