Erick...pick a club to practice, lets say 5 iron.
Setup with a ball and that 5 iron so that you are in your setup position and ready to swing and strike the ball.
Now get out of your setup and put down a long piece of something (string, ruler, yardstick, etc) directly IN LINE with the ball. This is your "plane line."
Now, set back up to the ball except you are going to be using your flashlights. The drill is to point one end of the flashlights at the line ALWAYS unless you are parallel to it.
So the light will be shining at the plane line at setup, during the takeaway until the flashlight is parallel to the plane line in the popular "toe up half way back" position.
Then you will begin to raise the club up through the rest of your backswing and your goal is to get the other end of that flashlight to, again, point at the plane line until you get to parallel.
HOWEVER
if you DON'T get to parallel the flashlight should still be pointing at an "extension" of the plane line ok?
Once you begin your downswing you will again use the one end of the laser to trace the plane line all the way to impact, both arms straight and then about the point follow through and swivel the other end of that flashlight should be pointing at the line.
This is how you trace a straight plane line.