If there is a large hole
through the wall, patching plaster will not stick. Two
methods can be used:
Method 1:
Cut a piece of metal plaster lath or heavy screen
larger than the hole.
Fasten one or two wire ties to the lath and insert
through the hole.
Pull the lath up behind hole and wire to a stick of
wood placed across the hole on the front side of the wall.
Moisten the edge of the hole and spread a base coat of
patching plaster to within 1/8" of the finished surface.
Roughen the surface with a stick or nail so the finish
coat will stick.
After the base layer is dry, remove the stick and clip
the wires close to the surface.
Trowel on the finish layer being sure it is flush with
the wall surface.
Let it dry well and then paint.
If the patched area is large, apply a coat of sizing
before painting.
Method 2:
Mark a rectangle on the wall which includes the hole.
With a knife or key hole saw cut out the rectangle
being careful not to break it.
On a scrap piece of plaster-board, copy the section of
the wall.
Then mark a frame about 2" wide around the section of
wall.
After cutting, remove the paper on one side and the
plaster from the frame around the patch. The 2" frame of
paper will stick out like an ear all around which will
hold the patch in place.
Spread patching plaster around the edge of the hold and
around the edge of the patch. Put the patch in the hole
and press the paper ears into the plaster and hold it
while it sets.
Cut a piece of metal plaster lath or heavy screen
larger than the hole. 
Fasten one or two wire ties to the lath and insert
through the hole. 
Pull the lath up behind hole and wire to a stick of
wood placed across the hole on the front side of the wall.
Moisten the edge of the hole and spread a base coat of
patching plaster to within 1/8" of the finished surface.
Roughen the surface with a stick or nail so the finish
coat will stick. 
After the base layer is dry, remove the stick and clip
the wires close to the surface. 
Trowel on the finish layer being sure it is flush with
the wall surface. 
Let it dry well and then paint. 
If the patched area is large, apply a coat of sizing
before painting.