Basic Zombie Makeup
Here is rule number one of zombie makeup.
You can always add just a bit, but you can't remove just a bit
This is a cliche I've espoused elsewhere but it's true. There's nothing worse than having the perfect zombie look, then going overboard with the brown greasepaint. So you have to compensate with the green greaspaint. And some more white to tone down. Then it looks wrong, so you try to rub it. And you smudge, and it's totally ruined. You get the picture.
There are two schools of thought on Zombie makeup, both of which have their own merits. One is the classic over-the-top 'Lucio Fulci' Zombi-style. Loads of green, deeply sunken eyes, and ooze. Plenty of ooze. This look lends itself well to a costume with huge amounts of gory effects, or the 'rotting' look, but some people think it's too comical. Usually this will be accompanied by very exaggerated acting and loud cries of "braaaains".
The other is the subtle version - the 'realistic zombie'. Just slightly paler than normal, perhaps a tinge of green. This is actually harder to do from a makeup perspective, as you have less to work with to cover any rough edges on your effects, but can have a much greater shock value, as people may not see that it's a costume from a distance.
My general technique is to aim for the second, and if you mess something up, you can always revert to the first.
There are two main things you can use for the base layer of makeup.
The first is white cream makeup, the sort clowns wear, available from any good costume store. Applying this you should always use a soft sponge and use long strokes so that you do not streak. As it is very creamy, too much of this may prevent further makeup from holding properly. It will also not take too well on top of latex or gelatin, in which case you want the second option which is...
Powder. I wouldn't recommend you steal your sister's $80 Chanel powder compact. This will not go down well. At its cheapest level you can just use talcum powder. Costume stores also often stock a nice light green powder that can be dabbed on with a sponge for a gentle tinge.
Eyeshadow / Eyeliner
Eyes are a vital part of the look of a zombie. They normally make people look alive, so we need to do what we can to make them look dead/undead.
The prime thing for zombie eyes is the sunken appearance. Elsewhere I have a prosthetic method to make this, but it can be done simple with correct usage of dark eyeshadow or charcoal to make the eyesockets look deeper than they are. The most important part to apply is close to the nose, above and below. Apply slowly, and watch the effect of the different areas. Some of this will depend on the shape of your face. See other pages on this site for more detailed application photos.
If you are female or emo you probably already use eyeliner. If you aren't I recommend you find one of those two groups to assist you with this bit. It is probably unneccesary for the 'realistic' look, but can be very effective. It's basically using a pencil (an eyeliner pencil, not just your HB!) to draw at the very edge of your eyelids, right by the roots of your lashes. This can do interesting things to the look, but should be done carefully by the inexperienced (and with help) because its so close to your eyeball.
Greasepaint is as it sounds, greasy, oil-based paint. It normally comes in blocks, 'zombie sets' being red, green and black. It is best applied with a stiff sponge.
If you are going for the 'realistic' look, you will want to be very careful with greasepaint, and even if you aren't you should do tests first (back of your hand maybe) before you tackle your main costume.
Greaspaint in large quantities can get very hot and sweaty to wear, as it blocks your skins ability to breathe. It is also not ideal for extremely hot weather, as the grease can get rather slick and sticky. 'Setting' the paint with a dusting of powder afterwards can help prevent some of the deterioration, and should always be done.
The basic technique is 'stippling'. Gently load your sponge (or cloth if you have no sponge) by dabbing a block of paint, and then stipple by very lightly dabbing it against your skin. An even coating of paint on the sponge is important, and most of all, do not rub. This will result in smears which never look realistic. Or Fulcistic. Just wrong.
Errors with greasepaint are nearly impossible to undo. My best technique involves 'stippling' with a dry kitchen towel.
Use the greaspaint to add texture to your skin, and you can also do bruises with multi-layered stipples of red/yellow/blue/green paints. Start with the yellow and green, and do a slightly larger area that you do for the other colors. This border adds a very realistic look to the bruise.
If you have used tissue on a wound, and there is a clear edge, bruises are a great way to cover this.
If you want a graze, theatrical (read: clotting) blood on a comb, scraped across the face, will dry into very convincing roadburn.