Not a method I'd ever recommend to anybody but depression did it. Just stopped eating, like, almost entirely, had no appetite whatsoever, would force myself to eat at least something around dinner time, around 50g of carbs (when dry) like pasta, rice or noodles. Drank tea during the day for some caffeine. Combined with some exercise - started walking then running about 5k every few days.

Things got a bit more normal after a while and just kind of went with watching calories. Mostly just kept an eye on carbs - no more than 100g per day, used less fat or oil in cooking, picked slightly (but not excessively) leaner cuts of meat, more veggie dishes, skimmed milk, no sugary drinks. Never was one for eating breakfast, my day would normally be some kind of lunch time thing like a couple of crumpets with some jam, an afternoon snack - usually rice cakes, japanese-style crackers, pickled stuff (gherkins, onions, sauerkraut) then dinner as I mentioned above. There was a few brands of ice cream that did low calorie versions I would buy for dessert, or I would have fat-free yoghurt and a couple of squares of chocolate.

I found this pretty easy to do during covid (started this all maybe mid 2020). It was easy to hide the fact you were eating strangely if people aren't aware. The bit that I found (and still find) hardest is the intention to start or cut portion sizes. I never intended to do it but I found that when I stopped eating because I had no appetite, it was like a kind of reset that allowed me to build up to a more appropriate diet. I can't say I think this is a good idea for a whole host of reasons but that is what happened to me.