After being an SCS member for many years, I know there are many stampers here who do something similar to what you do. I know that card sketches are very popular, and lots of folks make a "recipe" for their cards before they start to make them.
I, myself, am more inclined to start with the reason I need to make a card, like birthday, thinking of you, holiday, etc. and then go from there - but just in my head. I seldom write anything down, or draw it out.
I find an image I like, then decide on what colors I will use for the card, and the sentiment I will use. Lately, I've been making more sentiments on my computer than stamping them.
The design for the card comes to me as I work with all the pieces in the puzzle.
I think it's wonderful that all of us find different ways to be creative and do what makes us happy!