What WarHunterX said is true. If you haven't made any other type of games such as pong, breakout, galaga, etc. Then it would be best to start with those. The reason for this is that everything you learn from making those basic games can be almost directly applied to making a bigger game like ff. For instance a main menu in a breakout style game isn't going to be much different from a main menu in ff. The artwork and options are probably the only things you would need to change.
Definitly start with a design doc. I know you've been on the forums a while and I'm sort of a newb also so I'm in the same place you are. Good luck
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