Well, it depends on what you are willing to do to achieve it. There’s the IFBB Pro diet, which I wouldn’t recommend nor would I write it down here. There’s your friendly neighborhood crackhead diet, which I would direct you to the IFBB pro instead. Then there’s the extreme caloric deprivation.
Joking aside; its all about calories as you know, Calories in vs Calories out.
If you are willing to take the potential health risk, in terms of lethargy and lack of mental focus and such; a water fast will get you there the farthest, but unless you are able to remain at home for a long period of time, to avoid potential dangers of dehydration, lack of focus, low blood sugar; then I will recommend that you avoid it.
If you are going to take a fast, be it a week long, or as long as you can; then I will suggest ensuring that you supplement with Magnesium, Salt and Pottasium - A proper electrolyte powder is helpful, avoid Gatorade its useless at most. Its also known as Snake Juice.
Note: you will feel like hell.
Instead, do an IF clean, lean, low carb diet – Meat, fish, poultry and salads/locarb vegetables. Ensuring that you have a proper caloric restriction. Keto, IF, OMAD, whatever the hell you want to follow does NOT negate thermodynamics. CALORIES IN < Calories OUT.
You can start with -500 caloric deficit for the first 3 - 7 days. Then push it up until you start feeling like hell, that’s when you know that you’ve entering the danger zone, and should preserve your energies and practice caution.
However, if keto and If or Omad is not your thing. Then drop everything aside and just focus on your calories intake. Nevertheless, ensure adding worthwhile calories and dont just do that cookie diet from that professor. Not all calories are equal. Protein. You don’t just want to look skinny, you still want to look healthy and not deflated.
Get Pysillium husk for your fiber intake and it clears off the intestines, helping you become more energetic and slimmer. If you do not have access, then SENNA LEAF will do fine, it comes as a tea or powder or pills. I think it was called Smooth Move if you are in Canada or The Untied States of Freedom. That’s kinda a cool name lol; no offense 
Get a proper multivitamin to be on the safe side.
Deload style weightlifting will be your friend; that is to say practice lifting less volume. The aim here is to preserve muscle mass and decelerate the muscle atrophy. The goal is not hypertrophy.
Taking a day or two, such as the weekends if you do not work or have outside responsibilities, for a full fast will be a helpful addition.
If you are willing to drop some coins, GLP1 antagonists would make the whole thing insanely easier, as it is apparent now by the stock prices of those Pharmaceutical companies
Stimulants such as Caffeine will be helpful to push through those days when you’re lethargic or just tired. I would suggest pills with theanine, so you have a proper measured dose. Green tea will also work. You can do more research into this area on your own. I’m not an advocate of the stimulants or diet pills, they can cause more harm than necessary.
Simethicone AkA Gas-X, Gas Free, or Free Gas etc… If you suffer from bloating and flatulance dues to gasses, take it on the days leading to your event.
Also, on the final week – Introduce back your calories if you went into the deep end of restriction. You do not want to look unhealthy and a walking zombie stick. Ensure that you take Magnesium and Psylium husk to empty your bowels and thus reduce your belly size.
MELATONIN, or other sleep aid. I suggest low dose melatonin in general; the increase in IGF secretion and restful sleep will be a tremendous aid to your health in general as well as the necessary rebuilding and restructuring of your body.
A physical shifting custom with MDFY NOW will be another aid. LOTS/Paragon Sleep + Fat burning module.
