Hi !
You are on the right track!
The diet plays a huge role in tummy fat !
Your exercise also must include core strengthening exercises .
The stability ball is excellent to target this area.
Try ball crunches in sets 4 of 20 reps. As well as reverse crunches in sets of 15 reps.
Should you require a more detailed personalised programme email me for my rates and options available.
Good luck!