1. Commit to drawing the portrait. This is the most important step because it forces you to ask yourself: Do I really want to make the effort to draw this person? You may not. And that's okay. The point here is to make sure that you have enough energy and enthusiasm and motivation to not only finish but to enjoy the process.
2. Choose a subject worthy of that commitment. Whether in person or via photograph or from memory, drawing a portrait requires spending a significant amount of time with someone's face. And a good face is rewarding to draw. What's a good face? You'll know when you find one.
3. Prepare. Are you going for realism? Caricature? Sketch? Black and White? Color? Abstract? Two dimensions or three? Are there any challenging aspects to the subject's face? Is there something specific about the subject that you want to capture? Do you have the supplies you need and a comfortable place to work? You don't have to have all the answers. But even thinking through these basics will allow you to approach your portrait with the confidence to solve the problems that arise.
4. Work from the general to the specific. Focus on shapes and proportions and, if you're ambitious, how light falls on the face. Heads, hairstyles, eyes, noses, lips they all have basic shapes. If needed, lightly sketch a grid or guidelines to help manage rendering the proportions.
5. Practice. To draw a portrait of anyone is to create something new, something the world has never seen. You may not like what you've come up with. That's normal. Your eyes have to adjust to this new thing. Give yourself time to get familiar with your portrait and discover its unique beauty. One way to aid your eyes in adjusting to the new portrait is to draw another. Then another and another. Eventually, when you look at a face, any face, you will only see its beauty.