There are a number of ways to achieve this, it depends on what condition the ceiling is in , is there any coving fitted, also how thick the artex is. if its alight patten, pva sealent and a skim finish will do it, a heavy pattern will require a bonding plaster coat prior to skim, and also you could overboard and skim, but be aware that artex produced in the70s/early 80s, contained asbestos, and scraping or sanding it releases it, and maybe harmful...