How do I organize a zombie walk?

Step 1 Pick a reason, a place, and a time. Try to schedule far enough in advance to make Step 2 successful.

Step 2 Spread the word and recruit zombies.

Step 3 Hope that people show up. People are flaky and will usually back out on you when it comes to this sort of thing. As a general rule, expect to get half the number of people who say they'll be there.