How do I sew the elastic on my ballet slippers?

I've been wearing dance paws to jazz and ballet but I just bought my first pair of ballet slippers, but I'm not sure how to sew on the 2 elastic straps. I know I'm suppose to criss-cross them but I don't know how to sew them to fit my legs. If you can give me some "How-tos" then it would be great. They're male ballet slippers by the way.