Counting calories. I used MyFitnessPal, but I’m sure there are other apps.
Figure out how many calories you need daily to maintain your current weight, depending on age/gender/height, and then subtract about 500 calories from that. This also depends on your stats; you don’t want to go below your minimum daily calories. You’ll be hungry and dizzy all the time, and your body will try to store extra fat because it thinks you’re starving.
You can eat whatever you want, as long as you stay under your daily calorie number. You might not be healthy (depending on your food choices), but you will lose weight.