The best way I've found is to get a good idea of what kind of story you want to tell: who the characters are, how they all fit into the story, what they each bring to the experience. From there, bulletpoint what takes them from the beginning of the story to the end, and what they have to go through to get there - that can be sidequests, if one so prefers, or little story missions in each town, or what have you. The best games take place in a world that makes sense, so don't forget that your characters had lives before the player got involved, and might have lives after the last boss goes down.
Also, seven games is a lot to juggle. The best and only advice I can give there is to take it slow, don't rush anything, and be aware that what you plan as seven games, might turn out to be two. Or five. Or twenty. Stories are living, organic things, and they grow and wither like any houseplant. Feed yours well, and see what blossoms. And most importantly, best of luck. It's not an easy thing to develop a story-based game.