How can I become underweight?

DONT DARE tell me its not healthy. This is what I want , I know what im doing okay so please no lecctures. I'm 5'6.5 and 138 lbs. I just wanna be underweight , skinny whatever. Im I eat much less and move a lot will I lose weight ? The starvation mode thing is crap , lots of people seem to lose weight by eating less , but will I ? And for me to be underweight how much weight should I lose and how long will it take ? thanks.