Had a request for this in one of my other content. I use GW paints.
Scab red (Red Gore is good too)
First, paint up the face how you want it.
Next, paint an area of Dark flesh where the whole scar is.
Next, highlight with Red gore all down the scar.
Next, highlight with blood red on the upper part of the scar, where sunlight would hit.
Next, water down the blood red to the point where the paint would dry almost transparent and lightly paint around the area of the scar. The bigger the area the more irritated or fresh it looks.
Lastly, water down scab red, like the blood red, and paint in a smaller area around the scar, giving it more depth. Paint a little on the scar too. The darker the area, the more it looks like it is about to start bleeding or the more it looks like it is new.
Next will be a tutorial on my Soul Drinker's purple armour.