Here are four different videos that explain how to measure by sighting with your pencil. Essentially, you are looking down your arm, and placing your pencil on top of the object you wish to draw.
Two main points you need to remember to start this technique....
1. Keep your arm straight, and outstretched fully. This allows you to have predictable results.
2. Only look with one eye (close the other one). If you are stretching out your right arm, then you only keep your right eye open. If you are using your left arm, then you only keep your left eye open.