a diet just = eat less calories. most idols seem to just eat smaller portions and of healthier foods instead of doing keto or whatever. most probably also do intermittent fasting, at least the gg interviews that ive seen whre they dont eat until after theyre done performing so their stomachs are flat as possible for the stage.
you cant exactly customize a diet for a weight loss goal. you just eat the calorie number you set and then keep doing it until the desired weight is reached. unless you mean weight loss in a specific time frame? then you calculate the lb number (3500 calories) and see how many you need to be in a deficit in by the date you need