Be yourself with her. Do not make you being a Muslim what your relationship is all about with your grandmother. Focus on all the other aspects of your relationship with her that were true before you became a Muslim. If you like to listen to her tell stories, ask her to share more of them. If you used to bake cookies together, continue to do so.
I see that you are from the USA. Your grandmother has probably only one view of Muslims right now, and it isn't likely to be very good. If you change to become something different than what she knew you to be like before, you will confirm some of her worst fears. But if she finds you to be the same grandchild that she had before, and that becoming a Muslim has done little to change who you are in your personal relationship with her (I am not talking about not living as a Muslim in terms of what is halaal or haraam, just your interpersonal relationship with your grandmother), then she is going to see that it isn't such a big deal.
If she is from a Christian fundamentalist background, this may take some time. But grandparents are generally very accepting of their grandchildren. So, give her that time, and remain who you have always been with her. Don't make a big deal about it with her, and it won't be as big of a deal for her.