After you put the yukata on, you have to straighten it to make sure it's even. Then tuck it in like so, and wrap the other part around your hip. [demonstrates imaginary tucking in and wrapping.] You have to make sure everything is straight and sits in place, and the yukata is just above your ankles before you tie the undersash and the obi.
[He scratches his head.] It's kind of complicated, so if you need help later whenever you put it on, just let me know.