Many older houses have walls and ceilings that are out of square or not level. If this is not too severe, coving should be fitted normally with any gaps filled with adhesive or filler.
Although there is some flexibility in most coving to help reduce gaps between wonky walls and ceilings, extreme cases will require you to cut the coving into several shorter lengths to follow the contours of the wall/ceiling, and then smooth and fill the joints with filler.
Or you can purchase specialist flexible coving made from rubber latex that can be bent to fit wonky walls/ceilings or around domes and bay windows.
