The biggest mistake people make is overestimating exercise and underestimating diet.
Fat loss is primarily controlled by your calorie intake. If your diet is not correct, no amount of exercise will fix it. You can burn calories through activity, but you also eat them back, nothing changes.
The most effective diet for fat loss is one that is high in protein, high in fiber, and relatively low in calories.
Protein is essential because it preserves muscle while you are in a calorie deficit. Without enough protein, your body will break down muscle, which slows your metabolism and makes you look worse. Protein also increases satiety, meaning you feel full for longer.
Fiber plays a similar role in controlling hunger. High-fiber foods take longer to digest and keep you full, which makes it easier to stay in a calorie deficit without constantly feeling hungry.
Calories are what ultimately determine fat loss. Even with perfect food choices, if you are eating too many calories, you will not lose fat. The goal is to create a sustainable calorie deficit while still getting enough nutrients.