A projector mount that allows your video projector to hang from the ceiling helps place the projector in an out-of-the-way spot where it can perfectly project on the wall. A plaster ceiling, however, can cause problems because the plaster alone is not strong enough to support the mount and projector. You must use the ceiling joists within the plaster ceiling to brace and support the mount. If the joists are too far apart for your mount to reach them, you must create a bridge to properly mount the projector to.
Choose a projector mount that will fit your projector. Weight is the most important factor, as the mount must be able to support your projector. Consult the vendor for the right mount that will support your projector.
Pick a spot on the ceiling where you want the projector and check the position of the joists (wood supports) within the ceiling above. This is much easier if the attic is above you, as the floor is likely open upstairs; otherwise, tap the ceiling and listen for the hollow sounds where the joists are separated.
Install the projector mount to the ceiling through the joists if possible. Position the mount so it lines up with a joist on each side. Apply the mount's screws into the joists as described by the mount's assembly instructions.
Mount a ledger board to the ceiling to use as a bridge in between the joists. If the ceiling is open from the upstairs, insert the board between the joists and screw it into the plaster from above. If not, screw the board to the ceiling through the joists from below. Attach the projector mount to the ceiling through the joists and/or ledger board.
Connect the projector to the mount as per its instructions. It mainly involves screwing the projector in place through its mounting holes.
Look over all of the projector mount's instructions before proceeding. Some, for example, may require that you install the projector to the mount first before mounting it to the ceiling.