My creative daughter made this costume herself. She used one of the little costume makeup kits available at Halloween and a tube of fake blood less than $5 in all).
We used a pair of black slacks and a black T-shirt.
1 - We used scissors to simulate the cuts/decay. The cuts are all diagonal. We laid the slacks on the table and drew a diagonal across one leg and continue to the other. This when cut, this produced a one-slice effect.
2 - After we finished drawing the cuts, we used a pair of fabric scissors to cut where we had drawn. Be careful not to cut through the front and back (that would make it hard to wear or make it come apart).
3 - We machine-washed the entire costume and machine dried it in order to get the cuts to curl and ravel just a little.
4 - On some fabrics, that will be enough. If that doesn't produce the aged look you want in the cuts, use a sharp knife or one leg of the scissors and scrape the fabric hard to give one or two places a frayed look. Then wash and dry it again.
5 - Put the costume on and drip the fake blood in all the cuts you made. Be careful here, a little goes a long way.
6 - We started by applying makeup with the black makeup. Trace the temples and around under the cheekbones with the black makeup. Use black makeup to make a rounded triangle on the end of the nose. Put black ovals around the eyes.
7 - Use white makeup to fill in the rest of the face following the bones of the face that you can actually feel and you can use a skull model from online to help measure the approximate contours of your face.
8 - Cover the whole mouth in white, then holding lips tightly shut go back over this with a line of black.
9 - The undead in one of our favorite movies has his mouth sewn shut "so he can't tell his secrets even in death" according to the plot. To accomplish this, we used the black paint to draw straight vertical lines across the lips above and below.