A rangefinder will basically tell you the distance to an object you want to know. If you are measuring the distance to a tree or golf pin then you just point it towards the object and check the distance. After that, you proceed with the rest of the steps in accordance with the information you just got.
Now, you may have practiced and practiced but could not quite get what you are doing so wrong that you are not getting the game right. Normally what happens in these cases is you learn to speculate your moves throughout the whole swing and then you realize that one thing you were doing wrong.