Before attempting the boomerang it's essential to be able to balance a top in hand. The better you can stabilize, the worse of a throw you can save. Use skyrocket, scoop, a launcher or whatever means you please to get the feel until it becomes natural.
Don't pull back too soon, make sure the top has flipped and started to spin. The entire motion must be smooth - as with a regular spinner any jerks will cause the tension in the string to drop off and the top to stop accelerating down the string. Pull up as well as back and just aim for the general space in front of yourself.