What didn't work was the button holes. My sewing machine doesn't have a button hole foot, and the pattern itself had instructions for another method of sewing them - the patch method. In principle, you sew a patch over the fabric of the shorts, where you want the button hole to be, mark out the button hole on the patch, sew a little rectangle round it, then cut the button hole and push the edges of the patch through it, fixing and finishing them on the inside.
My first mistake was not realising what a 'facing' was. You need a flap of fabric which lines the back of the placket and covers everything up neatly. I had already sewn this lining or 'facing' down by the time I started the button holes. So, the first attempt looked like this (excuse the blue thread and the graffiti - it was a practice run):
The these two pictures (above) show the buttonholes from the outside. All quite incorrectly done.
These two (above) show them from the inside. Without a 'facing', all I could do was stitch round it, but because the fabric is all in folds - having been pushed through the button hole - the stitching was impossible to do neatly.
Anyway, despite the button holes, the muslin knickers were quite good - and the resizing worked well.
I then tried again in a lovely polka dot cotton fabric. I nearly got the button holes right.
These (above) are they from the outside. You shouldn't be able to see anything through them apparently - they should just look like two welts, flush together. Still, an improvement on my last effort.
The inside (above) isn't quite so neat. Also, I miscalculated the width of the facing, so when I came to cover up the rear side of the button holes, there wasn't quite enough fabric - leaving those little flaps you can see on the left of the photo.
Two other mistakes: firstly, the three button holes are actually all different sizes. Secondly, when I cut the pattern piece which the button holes were to be sewn on, I cut it out the wrong way round. I thought it wouldn't matter if I just flipped it over (it seemed almost symmetrical). However, of course it did matter, and that piece was 'taller' than the rest of the pieces. By the time I discovered this though, I had already done the button holes, so I couldn't just chop a bit off the top. So, there is an awkward fold of fabric just beneath the button placket, as you can see below.
The polka dot fabric, incidentally, was perfect. It was a lot thinner than the blue cherry fabric I used in the previous attempt, so the result was more 'underwear' than 'outerwear'.