It's never going to look absolutely brilliant, but, if it was me, I would stop the wallpaper at the point where the wall stops being vertical and starts to curve.
You could consider fitting a thin piece of wood, such as cover strip, to make an edge to paper to. Once fitted, the strip could painted the same colour as the ceiling, or the same as the rest of the woodwork.
Not very fashionable these days, but picture rail run a little way below where it curves would be my solution. Then paper to the rail, paint above the rail and over the ceiling.
I agree with maybe putting a thin rail.
If that's not an option you'd have to draw a line before the curve starts right across the wall and get a spirit level (or long metal ruler) and a sharp Stanley knife and cut along leaving a nice straight edge. Definitely before the curve though.
That's what I'd think about doing too. Or just paper up to a picture rail , and paint the wall above it and the ceiling one colour, or don't paper at all and paint wall and ceiling one colour. You could put some MDF panelling up to a picture rail height to add interest. We painted walls and ceiling the same colour ( duck egg) in our bathroom and it looks loads better than having a white ceiling when walls aren't white. The ceiling appears higher. Why do we have white ceilings anyway? Apart from anything else it's a PITA cutting in,especially if you haven't got a lovely even /straight angle where the 2 meet.
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