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We need a little more information.
How old is the house? More to the point is there wood lath existing behind where the wall was or is there a big void all the way to the roof and to the back of the wall? How much offset is there between the level of one side of the former wall and the other?
Your best bet is to hire a plasterer, but if you are set on doing this yourself then answer these questions and we can tell you how to do it. It isn't really compilicated but there are some steps that must be done.