Drink 2L water a day.
Eat protein and veg with small amounts of complex carbs, no refined sugar or refined flour products or UPFs.
Do an hour a day exercise minimum and at least 3-4 times a week, make this quite strenuous. Weight-lifting is fantastic. Body weight work is also brilliant. Flow yoga is multi-tasking as it burns a lot of calories, whilst toning you, stretch your muscles to keep them supple (which weight training absolutely doesn't) and also improves your posture.
Facial yoga (free workouts on line) tone up the double chins a bit. On the day, to take focus away from your chin, accentuate eye make up over lips, and style hair so it doesn't draw attention to the jawline.
A week before your birthday, get brows threaded, new cut and colour, a gentle facial. Maybe some light facial tanning oil too or a subtle spray tan. The day before get a mani and pedicure.
Buy a new outfit that is absolutely right for you in shape and colouring.
What shape, height, size and colouring are you? (Tall apple shaped brunette? Short pear shaped blonde? Fair skin, dark skin?) Loads of people on S&B here are good at finding perfect outfits for different shapes.