You will need the following:
Fake blood and blood gel (Gel is optional)
Flesh toned makeup to blend with
Dark purples, browns, and black makeup (Eye shadow preferably)
Apply liquid latex to the shoulder, Let it get tacky and add some toilet paper, add more liquid latex and then rip a small opening for the "wound"
Add flesh tones to blend
Paint the inside of the wound dark purple or black
Apply color as needed outside the wound
Add and blood gel as you see fit.
Now go and gross out somebody!