How is the best way to lose weight in a day?

You can't.

The only thing you can lose over one day is fluid weight, and not fat weight. And going around dehydrated is a very bad and unhealthy idea.

My opinion:everyone has to know that by now, but if they wanna know, they wanna know.

get fatigued ( early exertion pays off when cutting weight)

you can do this by:

-planks

-pushup position hold

do either until you cant hold it anymore

or, instead:

-Weightlifting inspired workouts...

(focus on the legs and do some of these: squats, deadlifts, jumprope, hip sled, jumping squats etc.)

then run 4- 5 miles

then drill or live wrestle

then hold plank or pushup position again (if you didnt do it the first time; hold for as long as possible

then drill or go live again

then go in a sauna for how ever long you like -.-

then weigh yourself

then do 25 situps

now your on weight!