Plastering the walls is actually pretty easy but if you need some help, see this great post. You will mix the plaster according to the directions. Once the plaster is mixed, apply two coats of plaster and then flatten the plaster using the trowels. You'll do this two times to ensure the plaster is smooth and doesn't have any cracks or other imperfections. The final step is to polish the walls and get them ready for painting.