Austin the best way to do it is just do it.Use the outside edge of youre thumb,hold it away from youre palm and use the edge like a knife to hit the string's.You need to hit them with a fast attach or else you will kill/dedend the string when you hit it.Secondly use youre middle finger to reach just under the string you want to snap pull it softly and then release but pull hard enough so the string hits the frets thus giveing you the snap sound you are wanting.Dont hit or pull to hard this is were most make there mistake it will sound sloppy and nasty not the clean hammer and snap that you are after. Best of luck JIM!
