First apply a layer of liquid latex to the wrist. It's up to you how big or small you want the wound, mine is quite large. Then carefully apply a piece of tissue paper to the latex. Make sure the edges of the paper are torn as this will blend better than straight edges.
Repeat this until you have 5-7 layers. Try to avoid making all of the pieces the same size. This will make it look like something stuck onto your arm. Try to leave the outer edges thinner and the centre thicker.
For the last few layers of latex I added a small amount of red and yellow acrylic paint to achieve a flesh tone. The colour didn't work out great with the acrylics so it's up to you whether to do that or not. Although it did create a nice dark ring around the wound.
Wait for latex to dry - 5 mins at the most.
Via Lost In Schlock.