How to bring it up?

Is your grandma the witty comments/advices type? If so, ask for her advice about which boys to avoid and which to keep near.

 

"The bravery of a potato." I'll have to remember that one.

 

One of the realizations that helped me the most when coming out, was the thought that there is really never going to be a "right time" to come out. Regardless of what you do or what you say, there is a very good chance it's going to be one of those hard awkward conversations. At least at first, and then what comes after is so much better, and so worth it. So just rip the bandage off. Just ask your grandma if you can talk to her about something, I know it's obvious enough and I'm sorry if it's not exactly the advice you were looking for. But from my own experience, any time I sat down someone important and made it clear to them how sure and important it was to me, not only did they take me the most serious, but they also appreciated the gesture. If you don't feel like you can get the words out of your mouth, just get them guessing. Just start by saying you have something important you want to tell them, but that's it's really hard for you to get through. If you start struggling, most people's reaction is to try and help you through it and they start to guess. You could also write a note and just sit with her as she reads it. I've never done something like that, but I know of a few people who have and it's worked out pretty well for them. Sorry I couldn't be any more help, I'm not exactly the creative type.

 

What? Potatoes are super brave! (*hug*)

Then, alternatively: either a letter like MrHojalata98 suggested, either wear a pride-theme wristband? Grandmas notice everything. (Everything.)
That way she might initiate the conversation, and if she does not, play with the wristband a little.

 

Good luck, it's gonna be fine!