I have finally written down some proper instructions to explain the process. The main instructions show how to make my default style which has gathered sleeves that sit on the shoulder and knitted sides that bring the shirt in at the waistline in gathers and drape fabric across the tummy and derriere.
I've included loads of variations at the end so that you can do different sleeves or give the shirt an empire line waist.
If you're doing my default version and it doesn't look quite right when you've finished, you can always unfasten and re-tighten the shoulders, and unfasten and re-knit the sides (even cutting larger sections before re-knitting if you didn't take it in enough). The finished version should automatically come in at the waist when laid flat.
PDF instructions
The witch is my own graphic design but it's up on redbubble if anyone wants it.