You'll lose if you eat less than 1688 calories averaged over the week, the less calories the quicker you'll lose but it will also be harder to maintain the diet because you are more likely to feel hungry/deprived so more likely to cheat. You need to find the right balance for you.
In terms of how to stick to 1200, make sure you have plenty of lean protein which will be lower in calories but will fill you up. Bulk out meals with lots of green/low calorie vegetables.
Things like broccoli, green beans, mushrooms, baby corn, cauliflower, general rule is veg that grows above ground is lower in calories.
Also don't drink your calories so stick to water, black tea/coffee and limit sugar and milk as those end up being calories you don't appreciate or feel fulfilled from having. Make sure you stay hydrated though, it stops you feeling as hungry and your body needs water to heal muscles that are getting more exercise and to break down fat.
One of my favourite lunches when I'm dieting is a warm salad. I stick a mix of the veg mentioned above into a microwave steamer and cook for 3 minutes, then tear up 3 slices of wafer thin chicken, mix it in with the veg, a teaspoon of real butter and plenty of salt and pepper. You can have loads of veg, a cup full of mushrooms is about 20 calories, so you end up with a big plate of buttery veg with bits of chicken for under 200 calories.