I was 12 when I started taking lessons, so I might have been able to comprehend what my instructor was explaining better than a younger kid.
I was on a lunge line, and went through the same "Up, down, up, down, 1, 2, 1, 2". Then she made me count myself. I had the up/down rhythm figured out the second lesson I posted in.
As far as the diagonal, she basically explained that I should be rising when the front leg closest to the wall was stepping forward. She had me stare down at the shoulders and told me when to rise and sit until I could tell which leg I was rising with. After that, she would just remind me to "check my diagonal". Sometimes it was wrong, sometimes it wasn't. It was a good way to drill it into my head!