Side is easy, you can do that with a bowl gouge, or a spindle gouge if needed. It is the transition that is difficult, usually in the bottom of boxes, hence the name box scraper.
In essence approach the cut with the flute at 3 oclock, handle way over other side of the lathe bevel rubbing, slowly twist to about 2 oclock to bring the cutting edge in and follow the cut straight down in, get out BEFORE reaching the bottom rather than curving the cut to the centre remove the bottom probably with a steep (60° ish) bowl gouge which will give a cut straight across. then work on the junction a bit at a time until the curve is as tight as you want it.
I have only really used this cut for hollowing boxes, it is not a shape I like aesthetically, or to cut, it lacks the satisfaction you get from a nice elliptical curve.