They used oatmeal, gelatin, latex, and a lot of makeup to create this realistic look. There have been a number of scenes within the series that have characters who have missing jaws, lips, arms, etc. How the heck do they do that? Well here is bicycle girl, who is the first walker to be seen in the show. This is how they got her ready.
So imagine this production of bicycle girl takes about three hours. There are scenes within the series that have twenty to fifty walkers walking the streets. Imagine how much time that has to take making everyone look like a legitimate zombie. While it probably only takes them a half hour or so for the smaller characters having to do twenty to fifty of them is insane. Really appreciate the work that is done by makeup artists because they are extremely talented and creative. Also I totally recommend watching "The Walking Dead"! It is an amazing show!!!!