hi , my opinion is you must be able to picture it in your head !! this is crucial in my point of view if you can do that then you can jot down a pic on paper it doesnt have to be art but you should be able to get wahts in your head out even if only you can see it on paper !!! I stress this doesnt mean you must be a sketch artist but to be able to give yourself a bluprint of what you do :) you can also train your self to draw with polys though this is more dificult Dont get discouraged just keep sketching when you feel you can translate on paper wht is in your head then youll be fine most people think you have to be a 2d artist thats not true most concept art isnt that great but it conveys the idea that is all that is important hope that helps keep at it if you want it youll get it !