The best way to do this is to buy a sheet of 3/8" drywall. Even out the edges of the holes in the plaster, cut a piece of drywall the correct size, and attach the drywall to the lathing using drywall screws.
Next, tape the joints between the plaster and drywall patch (I'd use self-sticking fiberglass tape for this job) and then spackle using pre-mixed mud. Again, my personal preference for this job would be the extra-light, super-whipped joint compound, which has more air in the mix and doesn't shrink as much as regular joint compound.
The only tricky part of this job comes in applying the mud. You've got to get enough of it in the right places, using wide knives, to even out any differences between the old plaster -- which tends to be a bit wavy -- and the drywall, which will lie perfectly flat.
Good luck! firstname.lastname@example.org