there is wooden boards behind my drywall...
you just want a clean cut out for the drywall? i'm assuming you are just talking about cleaning up the edges of the drywall to get ready to install the repair piece.
just grab a straitedge (or chalkline) and a tape measure, measure out the piece you want out, trace it with the straitedge, score it with a knife (go lightly, several passes will be needed to cut through) it's not that easy a task for a ceiling if you're not used to it, but just keep at it.
You can use a sawzall too, just be careful not to cut right through the boards beneath. This will be a lot more dusty too.
Then you are ready to tape and mud. Make sure you prime this before you paint.
It sounds like you really don't have much experience with drywall- Some words of wisdom- It may be in your best interest to have someone come in and do the drywall repair for you- it is a task that takes a bit of practice to produce great results, especially on ceillings.
If you want to do it yourself, cool. Just be prepared to be patient and learn as you go, if it doesn't look good just sand it and mud it again, the farther out the better. Hard for me to tell as I have no idea how big the area you are talking about is
Last edited by chrisBC; 07-07-2011 at 01:51 AM.