As I was hemming a jacket, I remembered I have a couple of "down turn fellers" for my coverstitch machine - and it was time I actually used them. I normally bodge it (no surprise there then!) and just turn the hem under as I go, which mostly works, but using this gadget was brilliant, but I must change needles and use the right and centre, not the centre and left as it's left a bit of an edge on the inside, but I have scissors.
If you have a coverstitcher and don't want to spend ages preparing hems, then I recommend one of these