Add some guts and internal “stuff”. You can make guts by spraying long strips of spray foam onto a sheet of wax paper and peeling it up. Another method would be to use a sealant backer rod to form the guts and then apply a thin layer of spray foam over it. Snake the foam around to the desired look and attach to the body using hot glue or small dabs of spray foam. You can also make a stomach, lungs or other body parts by using the same method. Take a look at an internal anatomy chart for size and location of “stuff”.
To form tendons and muscles over the guts spread a thin layer of liquid latex onto a smooth flat surface and let it dry. Then just roll the layers into thin tendons and attach to the body using more liquid latex or hot glue. To make bigger / thicker tendons soak a piece of creepy cloth fabric or similar material in the liquid latex an then spread it across the area on the body where you want to install it.
Apply a base coat of exterior latex paint to the corpsed areas only. Most people use either black or dark gray however you can use a dark green if you want more of a zombie appearance.
Apply a drybrush (acrylic or latex) of khaki over the base paint to dull the dark base coat.
Next drybrush (acrylic or latex) on some pinks, light purples and red blending the colors together. Apply this only to the internal gut areas and make sure the base color still shows through in low areas. This will give it the “blood” coloring that you want.
Apply a light color stain like Minwax “Honey 272” or “Early American 230”. Dab on with a brush and pat dry with a cotton cloth. Apply additional coats to make it darker in some areas as required. Apply the stain over the entire prop including the plastic skeleton portion and wipe dry. The stain will blend all the colors together.
I wish I had some photos to show you but I don’t do much plastic corpsing, I’m more of a mache guy, but if you want to see a master at plastic corpsing then check out The Ragged Grin: The Ragged Grin