There is a comprehensive guide to drawing up the management plan for a green space, at the webarchive: here

This 48-page guide from CABE explains what the Green Flag awards are looking for, who the plan is for, the lifespan and revision of plans, and who should be consulted, amongst other things.  It also sets the expectation that it is written by a team at the local authority, but we have found that this task is increasingly delegated to the green space Friends, so this is why LFGN is providing resources for writing them.