the 4 arrivals halls at schiphol form 2 sides of a triangle around a central plaza that contains the railway station - see here http://www.schiphol.nl/Travellers/AtSchiphol/Maps/ArrivalsAndSchipholPlaza1.htm
buy tickets from the large ticket office or the many self-service machines, and the platforms are just 1 level beneath the plaza, connected to it by escalators.
As mushroom says, it's pretty easy - I did this just last Monday!! (Exhibition at the RAI). BUT be aware that the ticket machines in the plaza only take DUTCH debit cards or ANY credit cards and NO CASH OPTION! Spent ten minutes swearing at the bloody thing before a nice dutch girl explained it to me. It's only a couple of stops to Centraal Station from Schipol. Think it will be platforms 1&2, about every 10 minutes (lots of trains run via both Schipol and Centraal)
Yes, do not exit the airport arrivals external doors, unless you are going for a breath of fresh air first, buy the ticket at the machine or at the counter then walk to the train station, I did first time, no problem, Schipol is an easy airport