You can get great results and install cornice yourself by following  a few basic steps  - why not do it yourself? To install Plaster profiles you will need the following tools:   - 3 clean buckets - Sharp hand saw - Plaster profile mitre box - Cornice cement - Medium trowel and Hawk   For larger jobs use a longer setting cornice cement as this will give you more working time.
These come in 30min, 60min, and 120min.
Also make sure that the wall and ceiling joins have been set and sanded before you start.
Also it's a great idea to have another person ready to help you place your plaster profile into position.
  Measure and markStep one: Measure from wall to wall and cut the cornice to length using the cornice mitre box and hand saw.
For 90mm standard cornice measure down from the ceiling 90mm and make a mark or put a temporary nail half way in,  or better still use a chalk line to mark a line right around the room at 90mm down from the ceiling, so the cornice can sit of it when installed.
If you are using different sized cornice the principal is the same, just measure down the wall with measurements of the cornice.
Be careful with highly decorative cornice as the patterns will need to match up for it to look right.
  Mixing the cornice cement   Step two: Mix the cornice cement to the consistency of toothpaste, it should be sticky.
The right consistency will allow you to press the cornice onto the wall and ceiling and it will stay there without any nails or screws.
Place the cornice upside down on two of the buckets and butter the edges of the cornice along the complete length.
Please note: if you are installing heavy, fibrous plaster, decorative cornice you will need to screw it in as well to hold the weight while the cornice cement sets.
  Setting and cleaning   Step Three: Using an extra pair of hands press the cornice into place using the marks or the nails that you have placed in step one.
Press into place and clean the joints immediately with a wet sponge or rag.
Cornice cement is hard to sand so it is best to clean the joins while the cement is wet.
Add cornice cement to any gaps and remove that cement that has been over filled before it sets.
   For joins in larger lengths cut the end on a matching angle using the mitre box so that when plastered the join will not be seen.