What is it?
A manifesto is a public declaration of principles and intentions.
Why is it useful?
It is useful to communicate what you intend to do so that others can process it, rally behind you, be inspired by it or decide to write their own manifesto. Its a statement of intent which can pave the way for serrendipitous help and signal to others who have similar objectives so that you can work together, share ideas and resources.
How do I use it?
Google famous manifestos. Read them and digest the kind of thing they contain and then attempt to write your own. Pin it on a wall somewhere you can easily see it.