In this tutorial we'll create a blooding scar effect on a completely healty skin.
The first thing we need is a face (or any other piece of skin you want to apply the effect to).
I chose the photo of this girl from Sxc.hu.

Download the full size image HERE because we'll need to zoom in a lot (you need to be logged in to download the full size image, registration is free and fast).
If you don't want to create an account you can apply this effect to any other photo that depicts a piece of skin.
Pick the Lasso Tool, set the Feather to 0px and check antialiased.
Now make sure your zoom level is set to 100% and draw a selection similar to mine.

Now create a new layer, pick the Paint Bucket Tool, choose a dark red ( I'm using #9C0000 ) and fill the selection.
Pick the Burn Tool and set it to 4px round, then draw some overlapping vertical lines to mark the deeper zone of the scar.

Now zoom in to 1000%, scroll to the area of the scar and pick the Pen Tool, select the Paths mode and draw some stitches. You'll need to be able to use the pen tool to make this.

In case you had never used it before:
The first click makes the starting point, the second click the ending point, then you can adjust the curves holding ALT and moving the handlers near the anchor points with the mouse.
This is what I have now.

Now select the brush tool, click on the brushes menu and press the arrow in the top right corner, then select Reset Brushes.

Now select a 2px round brush.
Select black as foreground color, create a new layer, pick the Pen Tool again and right-click on the path, select Stroke Path, choose the Brush tool and make sure that Simulate Pressure is not checked.

Now right-click the path again and choose Delete Path and set this layer's opacity to 75-80%.

Create a new layer and place it below the stitches one, select a dark red as foreground color (I chose #3F0000) and pick the brush tool, select a small grungy/dusty/dirty brush and set its flow around 50%.

Draw over and around the deeper part of the scar to give it a dry appearance. Now hit Filter > Blur > Blur.

Now click on the base blood layer, select the Smoodge Tool with these settings and choose the color #9C0000.

Draw around the borders to give the red shape some irregularities where it's too straight.
Moreover you can draw some blood at the bottom.

The main tutorial is completed, now if you want you can go further and remove the wider bloody part and obtain another kind of scar, cleaner and more realistic.
Ducplicate the blood layer at the bottom and hide the original one (click on the eye at the left in the layers palette).
Pick the Eraser Tool, brush mode, Opacity and Flow 100%, zoom in to 300% and erase the blood you don't want.

or

If you want to have a darker red you can desaturate it a bit
Got to Image > Adjustments > Hue/Saturation.

and the result will be