I used to be an exercise/weights junkie "back in the day".

Best way to get your lower belly muscles is to lay flat on your back, arms outstretched and flat, palms down for balance. Keeping your feet together, lift your legs about 4-5 inches off the ground, hold, and slowly lower.

Its actually the lower slowly bit that works the muscle. The slower you lower, the more it will burn.

While you are lifting your legs, inhale.

While you are lowering your legs, exhale.

As you get stronger, and your balance gets better, you can do a few in a row without your feet touching the ground in between.

If you have trouble holding your feet togther, you can do a modified version by crossing your ankles, to help support your legs.

No doubt, when you start, it will be difficult, just do a few at a time so you dont hurt yourself, and get discouraged.

When you are done, lay with your arms outstetched over your head, breath deep and literally just SUCK IN YOUR BELLY, hold it, and release.

Keep yourself well hydrated, and use a firming moisturizer to help the skin regain some elasticity. I noticed a MARKED improvment in my skin and its elasticity since I started regularly using Nivea firming lotion.

Speaking of exercise, I met an old friend walking the other day. She's lost over 100lbs and is now a personal trainer. I asked her about situps and things that are difficult because of my weight gain, and she suggested starting with "side situps/crunches" lay flat, knees bent, keep one elbow down on the ground for support, and and lift the other towards my opposite knee. After a while this strengthens your stomache, and sides, and then doing regular situps/crunches will be easier and less stress on your back.

Good luck!