How do you lift weights for a flat stomach?

It's hard to explain on here... I have a subscription to Oxygen magazine and over the years have compiled a favourite list of ab exercises. Depending on which move it is, I would use anywhere from 5 to 10 pound weights. I would use whatever weight I could bear without putting strain on my back. Most of your typical ab exercises can be done cradling a weight on your chest, or in between your feet/knees. Also, take advantage of the benefits of doing some of your ab exercises on a yoga ball. The abdominal twist on the ball while holding a weight, russian twists while holding a weight, reverse crunch with weight in between feet, regular and side crunches with a weight on your chest.

Hey, and though i am still going through my personal trainer course one thing i do know is that toning is not a strength thing. I personally have never been muscularly defined, even when in the marines doing 500+ sit ups a day. What you're looking for is a balance between strength training, cardio workouts and a healthy diet. Again, I am neither a dietician nor a CPT, but i am in school for both. Keep tracking your meals, and working out. Eventually you'll start to notice a change. If all else fails reduce daily intake by 100-200 Cal per day and you might see the results then. Hope this helps and if i could help any further or you just need some encouraging words send me PM.
-Good luck