First, dust powder on the area. You can use translucent setting powder, or your normal face powder.

Next, use a lip pencil in a dark brownish-red color to draw your 'gash.' I like to use cheap pencils, like Wet'n'Wild or Jordana. Dollar store ones work great, too.

Now, brush a layer of rigid collodion over the color. Let it dry. Repeat this process about 4-5 times, until your gash is nice and deep.
As the collodion dries, it contracts your skin, creating a channel. The more layers, the deeper it is.

Now, carefully line the outside of the 'gash' with liquid latex. Let it dry. Do this repeatedly. You can use a hairdryer to speed up the drying process. It's very important to let the latex dry between coats, otherwise it will have a milky appearence. You can keep adding layers until you've built up some skin that looks like it's a bit peeled. Alot of times, I'll add more layers in certain spots than in others so that the skin is jagged and ripped looking.