There are two ways to draw portraits. You draw it straight or by using grid lines depending on what style you want to adopt.
If yours is to transfer your spirit into your work, you don't need to grid the format before starting. But if your purpose is to capture the resemblance, then you need to grid your format.
Next stage is sketching. But make sure you are neither scared nor too confident. Just try and be at the middle, and let your strokes be firm and sure even if there are errors. Those errors will soon disappear of themselves into the beauty of the work (hope you know what I mean) as you continue to draw.
Some shapes of the body forms may not come out by your application of line drawing but by the shading, so be careful not to draw out all the body forms in lines. (i.e. leave out contours and let them show by your shadings and not your lines).
Begin to draw out the basic shapes of the forms. This is applicable only to those not using grid lines. But if you sre using grid lines, follow the lines to guide you to draw. This technique is good for beginners but won't make your job look proffessional. An artist can easily know you used grid lines.
Lastly, go on and start bringing out the forms through the appropriate shading technique for each form.