As years go by and we grow older each year and wiser as we would like to think, there are certain statements that define our being. These are shaped by our values, beliefs and experiences.
If you allow yourself the time to ask yourself such questions, you would be in a position to come up with your creed or manifesto. You don't have to reinvent the wheel, though. You can start with a creed or manifesto that others put together. In time, you can adjust it, keep the parts that resonate with you and remove the ones that don't.
As Picasso had said,
Good artists copy, Great artists steal.