You can vibrate runes and affirm something like "I am losing the unwanted and excess loose skin in a heathy and positive manner" or use another affirmation that works for you. You can also use the extremely powerful SATANAS to add power to your affirmations.
However, getting rid of loose skin through magic will take an immense amount of energy. It may be a better option to have surgery to remove the loose skin after you have lost weight.