From Scratch to Scarface
To begin, Draw an Outline of the scar with the Lasso Tool. Don’t worry about making it perfect; a little unsteadiness of the hand will actually make it look more realistic later on. Hit Shift + Delete on the layer and fill the selection with black.
The give the scar its necessary Depth, create a New Layer and paint over the hard edges with a soft edged Clone Stamp. Don’t go all the way to the center because this will look like the Skin that is inside of the scar. Clip this layer to the underlying one by right clicking and choosing create Clipping Mask. Hit Ctrl+ M to bring up a Curves panel and make this layer much darker.
Next, on new Soft Light layer, use Black and White Brushes to dodge & burn and create some Depth and add realism to the scar.
On another layer set to an Overlay blending mode, continue to add some detail to the scar by painting with a dark red Color around it.
If you already have blood in an image, it is very helpful because you can Clone Stamp it from one section to another. In this particular image, we copy some of the blood from the nose onto the outside of the scar.
Finally, use the Blur Tool and make sure Sample All Layers is checked. Paint over top of the scar to blend it into the Skin. This is especially helpful if the image is not perfectly in focus.