Calories in vs calories out.
Literally nothing else matters, nor is it measurable or directly controllable outside of a physiology lab beyond a rough estimate. You can adjust for it indirectly later.
Weigh and log absolutely everything you eat. Every cup of tea, glass of water, boiled sweet and stick of gum.
This will not be a 10-day exercise. don't weigh yourself every day, there's too much variability from fluid shifts, hydration, defecation, pick the same time every week - i used to pick Thursday am, after a shower.
This is the bit where metabolism comes in - pick a desired weight and time to lose it - be realistic - I chose 20kgs over 9 months. Work out the daily calorie deficit - and stick to it. After a few weeks reassess - if your BMR and daily energy expenditure is lower than average - you'll be behind schedule, if it's higher you'll be ahead... Recalculate and work out how many fewer calories you need to eat, or how many extra you need to expend. Repeat, Repeat , Repeat.
Weight loss is not a target, for it to work, and to stick, it's a complete behavioural reprogramming.