Quarter-round molding is trim that's literally a quarter of a circle as you look at it from the end, with two flat adjacent sides forming a corner and a curved third side (like a slice of pie). It's routinely used to run along the wall at the floor, ceiling or other long span where it can fit snugly into the inside corner while facing the curved side out. Like crown molding, it can be complicated to hang on the ceiling, because you need to adjust the cuts to be upside down and backward on your miter saw.
Measure across the top of the first wall, from one corner to the end. Transfer the measurement to a piece of quarter-round moulding, with two marks along the top and the measured span between them.
Lay the moulding on a miter saw, positioned upside down and backward from the way it will be along the top of the wall. This means the left side of the trim (as it will hang on the wall) should be on the right side of the blade. One of the flat sides should be flush against the back rail of the saw and the other should be sitting flat on the platform, so the curved side is facing up at you.
Swivel the blade 45 degrees inward, to point at the middle of the trim. Make the cut.
Move the moulding so the second mark is under blade. Turn the blade 45 degrees in the opposite direction as it was before, so it's again pointing inward. Cut it.
Set the moulding to the top of the wall, with one flat side to the ceiling and one to the wall, so the curved side faces down at you. Nail it in place by shooting trim nails every foot or so, at a diagonal angle.
Measure the rest of the spans along the wall tops and cut them in the same manner. Nail them up the same way, so the mitered ends of the trim fit snugly to each other in the corners.