This is just my take on how to cosplay Rainbow Dash. I imagine her as a sporty pony, so I tried to reflect that. Sharpie dying a white wig is a boatload easier than making your own custom wig, just google 'how to sharpie dye a wig'. For the cutie mark, make an iron on transfer of it and place it either on the tank or the back of the hoodie. If you're in milder weather, you can substitute the tank for a raglan/other sporty t-shirt.