Wow, that is pretty much the worst advice ever.
All wearing a bag will do is make you sweat which makes you lose water weight. Water weight comes and goes and you may look slightly thinner with less water weight but as soon as you drink that "4L of water a day" you'll have it right back. (I'm not saying drinking lots of water is a bad thing.)
You need to set yourself up a meal plan that focuses around:
-lean protein foods (chicken, tuna, lean ground beef, etc)
-complex carbs (whole wheat pasta, oatmeal, etc)
-good fats (almonds, fish oils, extra virgin olive oil, peanut butter, etc)
Build a meal plan with 5-6 meals per day. (You didn't give any stats on yourself so I can't even guess as to how many cals but a safe starting point for a lot men would be ~2500 and women ~1500). Follow this meal plan for a week or two. If you have lost weight by the end of the week then great. If not, drop the cals by 200.
You have to find your "maintenance" level of calories and be a bit below that each day to lose weight.
You can also increase your body's maintenance level by doing cardio or weights. I would recommend both.