1800 Calorie Diet In 3 Weeks

I have to stay within the 1,800 calorie range of the program. When I read P90X's nutrition guide, it says there that the 1st three weeks should be fat shredding phase and in order to do that I have to minimize my calorie intake. Hard. but I can do it. It's my 3rd day in the program and I did Shoulder - Back and AB Ripper X afterwards.