what I think a best and fast way to become a designer can be to do a Diploma / Graduate course in from a reputed Fashion Institute and then to assist a Fashion Designer or work under a Designer with good name and fame. The Course can give you correct and fast knowledge about different techniques used by a Fashion Designer and working under a designer can give you the taste of their Experience. Both the combinations together can surely help you become a fashion designer. But the success will surely depend on the knowledge you will Gain and resources you have to fulfill your dreams.
Best of Luck.