TLBS,
Can't offer any direct advice etc as it's over thirty years since I had a go - but very good on you!
As far as keeping the "yaw string" in the middle - it's easy! Imagine another string running across the cockpit and joining the ends of your boots. Imagine the aircraft's "yaw string" is tied to the middle of your "boot-string".
As strings can't push, only pull, stretching your toes "pulls" the string back into the middle.
String out to the right? Pull it back by applying some left boot.
String to the left? Pull it back with right boot.
BTW, I flew over that "Last Black Shed" on Friday, on my way south!