Teaching Older Dogs To Fetch A Ball

Hi Everyone,
I am new to the forum and hope to find some tips regarding my problem. I adopted an abandoned American Staffie about 6 months ago. I think she is about 3 years old and looks to have had puppies at some stage. We have gone through the separation anxiety stage and she is now quite settled in our home. Even though we walk her twice a day in the park she has put on weight. I have tried to teach her to play with a ball but she appears to have no idea how to play with anything. Does anyone have any tips on teaching an older dog how to fetch ball?
Thanks Cherry