Magic wand and time machine!
Sorry, I'll assume the question was serious. The serious answer is there is NO long-term nonsurgical solution for flabby neck and double chin; even weight loss and exercise (facial exercises don't work, BTW) will only further "empy-out" the neck skin. If it's flabby now, losing weight will not make your skin regain lost tone, it will only make the laxity worse.
Surgery is the only real answer, and the exact type of surgery depends on careful examination of your exact anatomy. You may benefit from lower facelift/necklift, platysmaplasty with submental direct fat excision, and/or chin implant (augmentation genioplasty). Liposuction of the submental fat (even ultrasonic, power-assisted, or slim-lipo--laser-assisted) will probably not tighten your already-flabby skin enough to recommend any of these options.
Lose weight to a healthy plateau, then see an experienced, board-certified (ABPS) plastic surgeon for a consultation. You will appreciate the improvements that are possible!