Will you have luggage? Waiting for bags can take just as long as immigration/passport control (customs is not a lengthy process unless you are an importer). I'd check the recent timeliness of your flight to see if you're on a chronically untimely flight (though past pattern doesn't predict the future). If you're traveling during holiday season everything could take longer.
The train station is directly under the airport - follow signs to Schiphol Plaza and look for the train icon on the signs, you go down an escalator to get to the tracks. There are plenty of YouTube videos - I found this one with a simple Google search. If you are wanting to pre-purchase train tickets, I myself would allow 3 hours - you can stop and get breakfast and freshen up if everything goes quickly.
BTW you can check the level of expected airport busy-ness about three weeks ahead of your flight -
https://www.schiphol.nl/en/my-travel-day/today/arrivals