take busline 191/193/198 from Schiphol Plaza to Aalsmeerderweg. Takes about 15 minutes. From there you can walk a bit back to stand in the final approach.
Another option from the same busstop is to walk into the street on your left (Aalsmeerderweg) and continue walking in it for about 10 to 15 minutes. You will find the official spotter site on your left looking over the Kaagbaan. Rotation shots can be had here with a reasonable lense. A 747 full frame at around 50mm, a 737 full frame at around 90. Views are good from early morning till around 15:00, then the sun starts to face you.