Sand the surface of the old plaster with a 600-grit sanding block to remove the old adhesive left on the wall after removing the wallpaper. This will also scuff the old plaster, giving the new plaster a suitable surface for adhesion.
Wipe the wall down with a tack cloth to remove the dust left by sanding the wall.
Apply a 1/8-inch coat of new plaster over the original plaster. Fill in the divots left by the removal of the old wallpaper. Apply the plaster with a trowel. Get the wall as flat as possible.
Sand the wall with a 600-grit sanding block to smooth down the ridges left by the trowel.
Inspect the wall for any imperfections. Spot fill imperfections with plaster. Wait for the plaster to dry, then complete a final sanding to flatten the wall.
Place a coat of primer onto the wall with a roller. Essentially, you are dealing with a new wall so expect the first coat to soak up into the plaster. Apply a second coat if needed.
Apply a coat of latex paint to the wall. Use your standard painting method for painting a wall. Use a wide "W" motion to ensure you apply one even coat to the wall, then add a second coat in the same manner if needed, only after the first coat has dried.