Pretty much straight forward but can get confusing sometimes.

Step 1: Choose a Zombie

You will need
a large open space such as a park (do NOT play at night with small children)
a group of friends (15-25 people)
ONE person to be a zombie
everyone else is human

Step 2: How the Zombie(s) Must Act

A zombie can NOT run and must groan every so often (every, maybe, 2-3 minutes)
Once you are touched by a zombie you must become one (NO exceptions)

Step 3: How Humans Must Act

Act like a human (BE YOURSELF)
And RUN from the zombies

Step 4: How to Win

You win by being the last human
You may NOT arm yourself
If you become a zombie intentionally, that ruins the game for everyone else
Remember, safety in numbers does nothing except turn your group into zombies
Zombies, REMEMBER, groups of humans are good targets and groups of zombies work well

You may NOT pretend to be a zombie
THIS IS A SURVIVAL GAME DO NOT RUIN IT FOR EVERYONE ELSE PLAYING IT

Step 5: Suggested Playing Fields

Any large open area will do but I prefer
A park
A forest
An abandoned area with buildings
A campsite

Step 6: A New Piece for a More Interesting Game

I will now allow the use of MID- TO LOW-POWERED airsoft guns. Zombies can not use guns (obviously), but if you want a CHALLENGE, then allow zombies to use PISTOLS ONLY. As usual, you are a Zombie if you are touched and in this case, shot. ZOMBIES CANNOT DIE!!!!!!
If a Zombie is shot, he or she must stand still for 30 seconds. If getting shot confuses you (if it does, then, no offense, THERE IS SOME THING WRONG WITH YOU) then you can use paintball guns.