As a matter of practicality, administrators write policy, but your input is welcome. Indeed, you retain the power to veto and specifically change policy at any time, provided you can convince a consensus of other users that change is required.
Policy is written and published by administrators or those that they designate. The exact wording of policy matters is not usually placed before the community for approval. Instead, the rule becomes binding as is when published, and remains so until and unless further comment from the community decides to change specific wording within the rule, or to suspend the rule altogether.