Electrical outlets are usually mounted to a stud. Take the cover plate off to determine which side the stud is on. All other studs on that wall should be 16" from that one, edge to edge or center to center (16"o.c.)
If there are no outletes, make hole in the wall large enough to stick the end of a tape measure in. When the end of the tape hits a stud inside the wall, transfer that measurement onto the face of the wall. Again, all other studs should be 16"o.c. from that one. Some older houses might have the stud spacing @ 24" o.c., especially on interior partitian walls.
If the walls are fairly flat and smooth, you can glue the sheetrock onto the existing plaster wall and eliminate alot of the plastering over of your nails or screws. Alot of new sheetrock is hung like this on open framed walls to minimize screws and potential nail pops in the future. Ask your sheetrock supplier for the right type of adhesive to use on the plaster.
