I-cord cast on help?

I’m trying to start a scarf (gigi from Romantic hand knits) that uses an I-cord cast on. I’ve figured out how to work the cast on, but no matter what I do my first row of knitting is terribly loose. The glossary in the book says to practice it because the first row can be loose. Well, I’ve practiced, but mine is still terrible looking. Does anyone have any advice on what I might be doing wrong here? I am stumped!

Maybe you could just use a smaller needle for the first row…I used to use a larger needle to cast on before I learned to knit looser.