Starting from one corner, cut the miters.
Using a commercially available gypsum base adhesive, apply a generous layer to the flat back edges of moulding.
Press the moulding against the ceiling corner. Using a damp sponge clean the joint to smooth.
When joining the mouldings apply a generous layer of adhesive between the pieces. Clean excess adhesive using a putty knife. Once cured, sand joint smooth. If required, apply second coat.