It isn't a perfect science, more like a close encounter. Use something like MyFitnessPal where 99% of the foods are already counted, all you need to do is plug in approximatly how much you ate. If you have a smart phone, you just scan the package!

When doing this, always asume the worst, so if you eat a tuna salad, which you make yourself with 1 can of tuna and a glob of low-fat mayo, just assume 2 servings of tuna and a serving size of mayo. You may have used 1/2 the mayo but this way you overcount, sligthly under-consume your calories and the weight will start to come off!