What is you dream pair of sneakers ? or if you could design any pair of sneakers what would they look like ? After you have chosen or designed the sneaker you want to be it's time to start collecting materials.
Ideally you want to use things that you already have so you can recycle. I made my costume out of old white poster boards I had laying around, I think I used about 2 1/2 large sheets. I also had some pink paint, and come scarps of felt, a belt from a robe became the shoe strings and the only thing I really had to buy was gold paint.
Look though your closet ,look through your garage ,look though your friends closets and garages also.
If you are stumped for ideas I think this Jeremy Scott for Adidas sneaker
would make an excellent costume ! I should be this shoe...