Hair is where most portrait drawings fall apart. The outline gets drawn and filled in, the shading is scribbled back and forth, and the result looks like a wig. The fix is not talent. It is a sequence, and it is the same sequence for straight, wavy or curly hair.
The video is Kirsty Partridge's, and it is built as a do's and don'ts: she draws the mistakes first, then the corrections. The steps below are the corrections. Every image shows long, wavy hair around a blank face, drawn in ordinary graphite pencils on ordinary paper.
What you need
An HB, a B, and a 4B or 6B. A blending stump. A small, sharp eraser. Smooth paper. Realistic hair does not need expensive materials, and graphite is more forgiving than charcoal for a first attempt.
The five mistakes
Outlining and filling in. Hair has no outline. Sketch the clumps and their direction instead.
Back-and-forth scribbling. Every stroke should flow the way the hair flows. Scribbling leaves hard highlight edges and no depth.
Finger blending. Skin oil makes the paper greasy and the shading uneven. Use a stump or a brush.
Hard outlines around every clump. Clumps blend into each other. A solid line around each one looks like a cartoon.
Uniform lines and uniform highlights. Same thickness, same darkness, same spacing. Real hair varies constantly.
Clumps, not strands
The single idea underneath all eight steps: when you look at hair, you do not see individual strands, you see clumps with light on them. Draw the clumps, shade the gaps between them, lift the light off the top of each one, and the strands take care of themselves. Add a few stray hairs at the very end, not the beginning.
Where it goes wrong
It goes shiny. Too much pressure. Build tone in layers with a light hand.
It looks striped. The highlights are all the same. Vary length, brightness and spacing.
It looks flat. Not enough contrast between the deepest shadow and the brightest strand. Push the 6B darker and lift the highlight to bare paper.
The rest of the portrait
The same layered-graphite approach draws every feature. Lips and eyes use the same pencil grades and the same blend-then-lift sequence on a smaller area, and drawing the face gives you the proportions the hair sits on. New to pencil altogether? How to draw covers the grades, the light-source idea and basic shading.