How do I force my body into fat burning mode?

You cannot force fat burning, but you can optimize conditions for it. Create a calorie deficit by eating less than you burn. Eat adequate protein to preserve muscle. Do both cardio and strength training. Try intermittent fasting to extend periods without eating. Reduce refined carbs and sugar to keep insulin levels stable. Get quality sleep and manage stress to balance hormones. Stay hydrated. Be patient as fat burning happens gradually with consistent healthy habits.