Simply hanging something heavy on a plaster wall can result in damage to the wall and the object if you don't tap into the supporting structure behind the plaster. Though there are several methods of locating studs, or supports behind the wall, the most reliable is using a magnetic stud finder. Once you've done that, hanging a heavy object like a large mirror, television or work of art becomes a safer and simpler task.
Move the stud finder over the plaster wall across the spot where the heavy object will hang. When finder passes over a nail, the magnet will react. Mark the spot with a pencil.
Move the stud finder approximately 40 cm (16 inches) to the right or left of the mark. This is the standard spacing of boards behind a wall, but it can vary to as much as 60 cm (24 inches), depending on who built your house. Keep the stud finder moving until you find a second stud and mark that one as well.
Put a small bit into the drill, and drill into the first mark. If you keep getting resistance after drilling 13mm (1/2 inch), you've reached the stud. Put the drill in reverse, insert a bit slightly smaller than the width of the eye bolt and drill all the way into the stud. Back it out, and screw the eye bolt in.
Use a level to draw a line to the area of the second mark. The bubble in the middle of the level should be perfectly visible between the two vertical lines. Draw a horizontal line connecting the two marks with the pencil, drawing it along the level's top edge. Repeat Step 3 to install the second bolt.
Lift the object carefully, hanging it on the bolts. Stand back and adjust the hang as needed by moving the object slightly to the left or right.
If you do not feel safe lifting a heavy object, get a partner to assist you.
Be careful when drilling for studs. Sometimes a stud finder thinks it's found a stud when it's actually found a metal pipe or conduit. If you hear the shriek of metal on your drill bit, back it out quickly and start over in another spot.