N.B. flouff is a sillier word that floof and more accurately describes this gown.
So measure and check that you have enough trim/marabou to match your plans.
If you are using marabou, find a good box set to watch as you spend many hours hand sewing it on. I did so with a single stitch from the back, with multiple thread breaks so it wouldn't all fall down if one thread broke in the future.
I sewed 10 metres and then wrapped another ten metres plus onto it and then as a row above the first so it looked as fluffy as possible.
I added more marabou on the ends of my sash too.
Now the sash does generally hold the gown in place, but I also took a trick from vintage gowns and sewed an inner tie to the left front which goes under the right. And a corresponding tie at the right side seam. This means that the gown stays in place well.
I then sewed a large thread eye onto the right front which I loop the sash into. This isn't quite as big as my sash, because at this point, the sash is smaller to go into a knot to tie it. If you wish, you could also do this at the side seams, which mean the sash sits nicely and doesn't move around.
And now is the time to put it all on, arrange yourself on a chaise longue and recline in style!