We recommend first looking into signage ordinances and any pedestrian wayfinding standards in your community. You should also look into the requirements for applying for encroachments in the public right-of-way. (Initially, in Raleigh, our signs were not considered signs! They were considered encroachments, as they would not have been sanctioned otherwise.) We also suggest finding out who is responsible for public campaigns for pedestrian/biker safety and awareness in your municipality.
Luckily, we have found that a lot of different civic departments and interest groups might want to support Walk [Your City] campaigns! Folks have worked with urban design centers, BIDs, planning departments, public works departments, mayor's offices, parks and recreation departments, economic development departments, public art organizations, and a lot of different non-profit community development organizations. Think creatively and broadly to build support for your campaign!