keep my line parallel with the outer edge of the circle. then, figure out the middle of your top word. happy has five letters, so a p should go in the very center of the arc. from there, add the other letters so they're spaced evenly on the arc; use pencil so you can erase and re-do as needed.
birthday has an even 8 letters, so write a t and an h on either side of the center fold, then fill in the rest of the letters.
now for the fun part! go over your pencil letters with bright markers, and erase the pencil with a good, clean eraser. i did block 