Major wall damage on a plaster and lath wall, of the kind of damage that goes all the way through both the plaster and lath, usually requires that sections of the wall be removed and replaced. However, it's often just the plaster that's damaged, with the lath still in place. If the damage isn't more than a few square feet, you can repair it by replastering the lath rather than getting into pulling out and replacing big swaths of the wall.
Scrape off the loose or damaged plaster around the edge of the area to be repaired, using your putty knife, while leaving the exposed lath in place.
Spread patching plaster into the damaged area with your putty knife, putting the plaster directly on the exposed lath. Add as much plaster as is needed to fill the area, so it's almost (but not quite) level with the surrounding plaster. Use your 10-inch drywall knife to smooth the surface of the wet plaster.
Cut several strips of mesh drywall tape to the length of the damaged area, using a razor knife. Press the tape strips over the wet plaster surface, side by side, covering it completely. Don't let the tape overlap. Top the tape with additional plaster, in a thin layer, using your drywall knife. Let the plaster dry for a day.
Apply another thin layer of plaster, bringing the damaged areas almost level with the surrounding surface. Let the plaster dry for another day.
Apply a final layer of plaster, this time leveling it out completely with the surrounding wall and spreading the plaster with your drywall knife so it extends out a few inches around the damaged area and onto the surrounding wall. Let the plaster dry for another day.
Sand the plaster by hand, just enough to smooth it out. The repaired area is ready for repainting.