Step 2 - The First Layer
To start off, we'll open up our base in Photoshop and adjust the canvas to accomodate any hair that we are going to add.

First and foremost, we must determine how many "hair" layers we are going to need.  With this particular photo, since the hair is parted with one side overlapping the other, two layers will be needed.

Using the Polygonal Lasso tool, roughly select the area that will be the underlying hair.  



Keep in mind that the hair does have volume; it does not conform completely to the outline of the scalp.  Go to the select menu, click on modify, then choose smooth.  Set the value to 3 pixels.



Fill this area with your foreground color.



As of right now, this should look like some sort of strange shape, rather than strands of human hair.



Select the smudge tool.  The brush that you should choose is under "Natural Brushes" and is labeled "Charcoal 14 pixels".



Set the mode to normal and the strength to 70%.  Leave the boxes "Use All Layers" and "Finger Painting" UNCHECKED.



Begin to make strokes upwards along the bottom edges of the hair.



Continue until the definite edge of the shape has disappeared.