How to apply plaster to a plaster damaged drywall section
First of all, plaster and drywall are two very different things.  Please use the correct terms.
It appears from the photo that you have damaged drywall, that looks like it may have been improperly patched in the past.  While you could try to fix this using drywall compound, that will be a very difficult thing to make it stick and look good over such a large area.
The usual way to fix something like this is to REMOVE a section of the drywall, fit a new piece of drywall in the void, and then use drywall tape and drywall compound to blend the edges into the existing wall surface.