I did the same thing at my dear Little Nans funeral a couple of months ago.
I basically talked about the little funny memories all the granchildren had of her. She used to look after the five of us on a Saturday night and also on New Years Eves so our parents could have some fun. The speech was added to by each of us, but I read the whole thing, 4 sides of A4!
It was a bit daunting at first but I practiced it lots, I was wobbly for the first sentence then I took a deep breath, and it was fine, and I'm a real softy.
Everyone said it was a lovely speech just as they knew her and I even made people laugh, which was good as Nan would have enjoyed that.
You should keep it as a personal remeberance of her, that way it will all be heartfelt, just talk of your positive and fun times, and how things she taught you you can now share with your own children. I was taught cartwheels down the hallway, when she was in her 50's.
Hope it all goes well for you, I felt so proud when I had finished and I know Little Nan would have really appreciated it.