Place the index finger of your playing/picking hand (in this case, my right hand) on the D string at fret 15, and the middle finger of your left hand on the A string at fret 3, and press hard with your index finger and wiggle it up and down so that it just nudges the A and G strings. 

drop D (DADGBe)
~ stands for wiggle


  E |-------------------------------|----------------------------------|
  B |-------------------------------|----------------------------------|
  G |---0-----2---------------------|----------------------------------|
  D |--~15~--~14~----0-------0------|----------------------------------|
  A |---3-----0-----~15~----~14~----|----------------------------------|
  D |----------------3-------5------|----------------------------------|