Safety First - Organise an Electrician!
Have an electrician isolate light fittings before drilling a hole through the centre of the ceiling centre to push the light fitting wiring through.
If ceiling is painted you can scuff with 120grit sandpaper and lightly score area where ceiling centre will cover with a stanley knife or alternatively you can paint bondcrete onto area to ensure cornice cement will adhere. If ceiling is new no preparation is necessary.
Make sure all surfaces to be glued are clean and free of flakey paint or loose material.
Mix dry cornice adhesive to the instructions on the packaging.
Apply cornice adhesive around back perimeter of ceiling centre. Be careful not to apply too much so it does not creep out from under the edge of the ceiling centre.
Push the ceiling centre into the required position on the ceiling and hold for a few minutes.
Drive two or three screws of suitable length through the ceiling centre into ceiling joist or batten and prop the ceiling centre from the floor to hold into position until adhesive sets.
When propping use a soft surface to contact the ceiling centre face to eliminate surface damage. An old towel or sponge is ideal.
Wipe / scrape off excess cornice adhesive with a sponge, brush or small tool.
Once dry must be painted with sealer undercoat, then it can be painted any colour.