I had to trace this picture of my drawing in Photoshop because the pencil lines were too light, so I'm sorry that it doesn't look as elegant, but it's good enough to show you my point. This is the first and most important part of your painting. Your drawing is the foundation for your entire painting . If your drawing is rushed and not thoroughly checked before moving to the next step then every step after will suffer, and in the end all you really have is a hard learned lesson. Believe me, I have been known to jump the gun and start painting prematurely, and I have learned the hard way that doing so has never been helpful. Fixing a drawing while you are painting really sucks! For instance you may spend hours painting a hand only to realize that the forearm is too long, guess what you get to do now? That's right, start all over, this isn't Photoshop you can't just cut and paste the hand on a new layer and move it up; you must start again. The drawing could have been fixed in minutes, but jumping the gun can and will set you back hours or even days. This is not to say that your drawing must be a complete rendering of your subject; after all, our finished product is going to be a painting, not a drawing.
Your drawing should be an accurate map of your subject. I myself work from photographs (The photo I worked from was withheld per my models wishes). Working from A photograph, assuming it is a good one, can be very helpful! With a photo it is very easy to use comparison to check your drawing, anatomy, angels, etc. Once you get one thing right in your drawing everything else gets a lot easier to define. For instance, let's say you are absolutely sure that you drew the right arm dead on (Her right not yours). Now that you have that, you can check the rest of your drawing against it. Let's say that next you want to see how far out to put the left shoulder. In this example (on my photograph) I measured the width of the shoulders and compared that distance against the length of the right arm. The shoulder width is equal to the right arm from atop the right shoulder to just past the elbow. Next I measured (on my drawing) from atop the right shoulder to just past the elbow, made note of that distance, then made a mark for the left shoulder that same distance horizontally from the right shoulder to where the left shoulder should be. Now I have more information to use to find further distances. Eventually you will have many accurate markings giving you the rough shape of your figure and since you were comparing everything against each other your drawing should be accurate.
The next important part of your drawing is to make note of the different shapes of light and dark falling on the form of your subject. This will help you in the next step of your painting, affectionately known as the first pass. If you were to jump into the painting with just the body's outline, without notations of transitions in light and dark you would run into trouble really fast. The more shapes that you define in this step, the easier time you will have in the next step. If you are new to this I would suggest making as many notes on changes in light and dark as you can stand. In this example (above) I have just marked the major transitions. Note how some lines are sketchy, telling me that it's a more subtle transition in that area. Remember that your drawing is a map for yourself, so develop your own rules, just make sure that you stick with them and know what they mean.
Check that drawing once more before moving on to the next step, this is your final warning.