You'll need some basis in the card manipulation already - the more, the merrier.
Some people do variations of other flourishes and then combine them together, some will, as already mentioned, "fiddle" with cards and come up with new ideas, some will have the general idea of how the flourish should look like before they even pick the deck up, a lot of flourishes are created due to completely failed attempts at learning other flourishes, etc.
My personal opinion is that if you feel the urge to ask how to make a flourish, you're not ready yet. Just pick your deck up and practice some moves, it will come on its own sooner or later.
Good luck!