Hi Ettiene, this video will show you how - https://www.youtube.com/watch?v=0rYs3KI3Xao
There are several videos that I have listed that give a great deal of useful information on how to do this. I wish the best on your project!
Do you know what is causing the sagging?
Is it in the centre of the ceiling, or at the join lines with the walls?
Hi ....the join lines the wall and I have looked it is not water damage. The nails seems to just pulled away from the rafters.
Flipturn is correct. Where is the ceiling sagging and do you know what is causing it. No sense in fixing it if it is going to happen again because the problem exists that caused it to sag in the first place. Did it get wet, need to fix the leak. Has the house shifted so that the rafters are no longer level? What type of ceiling is it? Plaster or sheetrock? If it is plaster it is more complicated to repair, if it is sheetrock you can replace the sagging piece with a new one. Key though is finding out why it is sagging and then go from there. All the videos and information that Michelle and Betty provided can help you. Note that if it is a big area damaged by water you may need to call a professional to replace the damaged area. Sheetrock is heavy and hard to handle above your head. Good luck.