Welcome, Jenn! I'm glad you like the site! Thanks for the comments!
I haven't done it before, but I would think that you would just poke the needle through the middle of a stitch on the hat, and pull up the yarn from the inside of the hat. It would be similar to how you pick up stitches along the side edge of the work. (See video in the Advanced Techniques section.) Just draw an imaginary line on the hat, where the shape of the ears would go, and pick up a stitch this way, in each stitch along that imaginary line.
Hope that helps.