Drawing realistic portraits is a skill that takes time and practice to develop. Here are some general steps that can help you get started:

1. Pick a reference or photograph. Make sure whichever picture(s) you choose, drawing them matches your skill level.
2. Create a general outline of the face and head. Use a lighter pencil, use a 2H, or if you don't have pencils with a different lead, use a mechanical pencil. These pencils create thinner, lighter lines, which will be easier to erase if you need to change the outline.
3. Don't assume anything. Pay attention to whether the person's head is tilted or turned.
4. Begin shadowing. Use a darker pencil to create shadows and add depth to the portrait.
5. Use keen observational skills. Pay attention to the details of the face and try to capture them as accurately as possible.
6. Be patient and take your time.

Here is a video tutorial that can help you learn how to draw realistic portraits step by step: