I haven’t tried this hemming method yet (will do so on a friend’s kid’s jeans soon though!), but I used to have indigo trouser-fit jeans that I put in a “fold crease” down the front, like on men’s trousers – looks great when paired with heels/boots, which always takes up some of that slack, so I always have 2 lengths of jeans going on anyway, heels and flats. I could get the crease out of any well-made wider/flared jeans pretty easily:
1) Choose a crease line I want (or in this case, go with the new hem work)
2) Iron the dickens out of it to set that line
3) THEN after each washing:
• straight out of the washer, refold the WET crease you want, PRESS it, and fully air dry flat (easiest in dry months),
• or PARTLY dry in dryer (rainy days/months) and do the same – re-fold/press crease and finish drying flat.

Same concept for any bend in the fabric, whether it runs vertically or horizontally.

Oh, I just remembered: it doesn’t work as well on synthetics & synth-fabric blends, so if the fabric isn’t of 100% natural fibers (cotton/wool/linen), then the press-wet/damp-fold-then-air-dry method doesn’t hold nearly as well.
So, in synthetic/blend cases—or if in a drying rush, well…happy ironing.

PS–sorry for the lengthy comment! I never realized how hard it is to explain something that’s easier shown…now I understand why a picture says a thousand words!