Focus completely on the skull. If you study the skull you are going to notice all sorts of bone structure you typically do not see when looking at an individual's face, including ridges over the eye sockets, the indention of the jaw and the formation of the teeth. After learning to draw the skull, your portraits will be more realistic.
You can use the human head as a measurement stick for the rest of the body. The average human's height equals about 7.5 heads from head to foot (the measurement adjusts slightly from person to person, but this is the measurement for Leonardo da Vinci's Vitruvian Man). After studying the skeleton, you are able to measure the entire body without worrying that the legs are going to appear too short or the torso too elongated.
Accurately drawing a full skeleton body requires time. While drawing the full body it may be a good idea to draw a series of quick, rough sketches of the skull next to the body. This helps you keep track of the body length and ensure you are accurately drawing the skeleton proportions. The amount of time it takes to complete this task varies, depending on how fast the student works. You can expect to spend at least several hours on the project.
Taking hours to draw a skeleton is one thing, but drawing a sketch of the skeleton is another. Take anywhere from five to 20 minutes to complete a quick drawing of the skeleton. This is not a detailed drawing, just a sketch to capture the basic position and movement of the model. Practicing this is going to help you draw human sketches faster. After the time period elapses, move to a different position and draw the skeleton at a different angle for the same time period.