I just did a 2-sneaker cake (check my photos). You need to make a template for the cake - I don't know how big you want it. You can trace a real sneaker onto scrap paper. Cut out a piece of cardboard the size if the template; this will go under the cake.
You then bake sheet cakes and cut and fill them to make a rectangular layer cake a little wider than and as long as the template. Then it comes down to carving the cake to look like a sneaker. Having a shoe or photos near you helps.
Any more specific questions? I'm happy to help.