With what part of the image I'm going to use this brush for, I create a hard brush tip with a diameter of 5, an angle of about 40%, Roundness of about 40%. On the Shape Dynamic option, I click on Control (1) and select Fade.

I set the fade to about 120 (2), with a minimum diameter of 30%.

Settings like the above can differ from image to image, and from different parts of your illustration, so the key to effectively use this technique is to know what you need for your digital art.

Once the brush is set, I go and select the Pen tool (P) again , and create a path that goes from right to left using the pencils as a guide to its shape and length.

Right click on the path, and select Stroke Path from the selection box.

Each part of the image may need similar or different size and shape of brush.

By using this technique and employing different settings on the fade amount, brush size, angle and roundness, I'm able to vary the strokes for a more 'natural' look.

The result so far is seen on the left.