PommyMike said..
When you say one of the newb errors is letting the kite get too close to the side edge of the window, is that only controlled by board direction or is there much you can do with the bar and kite?
Excellent question.
I think this sounds like "newbie fear", which is where beginners are too scared diving the kite
hard, especially in moderate winds. To me 15knots is moderate.
If you could pick a more higher wind day, you will learn far more quickly and progress with the upwind skills.
It also depends on the kite, brand, board size, your weight, etc non of which was provided.
But not diving the kite into the power zone properly is a common noob mistake. Don't take offence. I was a noob once too.
Bring the kite back to 11 or 10 on the opposite side and just dive her into the power zone. You'll most likely get yanked off the board, but that's ok. Lean back and let the kite lift you up. don't try and stand up yourself. Keep that board poiting towards the kite (or downwind) and keep tension on your front lines (the ones from the harness). No tension says to the kite - depower, depower!
Practice.