First, look at the figure. Remember that the most important thing in gesture drawing, like contour drawing, is careful observation. Find the areas of tension, or the areas that are the most complex, like folded arms, twisted torso, oustretched limbs, etc. Once you have noted these, start drawing lines that replicate the shape of the area that you are drawing.
Move around the page at 5-6 second intervals. This will insure that you are able to capture all of the figure in a short span of time. For example, if you are working on the left shoulder, force youself to stop and start working on the right foot, etc. Keep the speed slow, concentrating on specific details, rather than fast movements.
The end result will seem disjointed. Since you are moving around the page, you have few clues as to where individual lines should fall. You will find that with practice, you will be able to place all the individual elements in places where they seem correct and your drawings will improve in accuracy and speed.
The image above is a gesture drawing of a male model. This is a quick drawing because it uses less lines and is drawn from efficient observation. There are openings in the outline of the figure. Leaving more white space is a good way to be efficient and makes a more interesting drawing. Gesture drawings can be done in squiggly or scratchy lines, but should be based on careful observation and focus on the areas of "tension."