I assume your walls have a texture at present. Depending on what kind of texure it is, you may scrape/sand as much of it as you can off, but if it was original to the plaster your best bet is to apply new material over it, in successive layers, until you have a smooth finish which you would then have to sand.
You could use joint compound, but I suggest a quick setting joint compound like Durabond, with a setting time of 60 minutes. You apply it with a 10-12' trowel.
It may take three coats or more, applying it in such a way as to fill the `valleys' caused by your trowel hitting the original wall texure.
You could then simply sand it, once it has set and dried, with a sanding pole.
For more complete information, contact me at my e-mail address.
Estate Plaster, Inc.