To start out, lightly sketch in these simple shapes. These shapes will guide us in building the body. Rearrange the arms and legs for some variety.
On the right of the circle you've drawn flatten it out just a little for the face. Afterwords, draw your zombie some tattered clothing. I started here by giving the zombie a torn up jacket and frayed pants. I built myself guides for his hands and feet as well.
Add even more detail here. My zombie has two patches of hair on either side of his head. Keep the hair limp and dry looking. On the very top of his head I've cut out a triangle, leaving his brain exposed. I used round circles for his eyes, but opened them only enough to see a small slant of his eye. This will give him a tired out look. If your zombie looks like its missing something add some bandages to his arms or legs. Here Ive chosen to have them coming unraveled.
Time for finishing details. Here I added fingertips to the hands, and sharp teeth along his mouth line. I also added some lines for his sharp cheekbones and stitches along his forehead and chest. I chose to add some tattered eyebrows and some deep wrinkles under his eyes.
Look at what you're left with. At this time decide where more details need to be added or where some details could be subtracted. Once you've done that, pull out a clean sheet of paper and trace this image over keeping only the lines you've chosen to keep.
So, having trouble making your zombie look old and decaying? Think back to a time where you maybe left something too long in the fridge or found fallen fruit rotting next to a tree. Pretty gross, right? Think about what made that image so unappealing to you. Wrinkles, mold, etc? Why not add that to your zombie? After all he's been in the ground a while, right? Keep in mind the colors too. Can you already see where I got much of my color pallet?
Small details can always make a difference.
See those small flies flying around him
Or what about the exposed bone in his leg? Have fun and try some of these tricks on your own zombies.