Okay, since you're not looking to focus on having visible abs and simply want a flat stomach, I have some recommendations. First off, you need to make sure your diet is in order. Nothing I say after this matters otherwise. Now, I recommend you incorporate jump roping. Multiple 3-5 minute reps. When you get up in the morning and before you eat, do some stomach vacuums. It takes a little practice but you'll get better over time and you will feel it if you're doing it properly, trust me. You want to do some reps of burpees (at your pace), you want to do some planks, I recommend donkey kicks (a favorite core exercise of mine, I do mine in the plank position). And add some reps of scissor kicks as well. I think these would be a great starting point to your goal. Again, the diet is first and foremost. You will not have results without eating right.
Hope this helps.