Welcome to the forum Carolyn! I've never made a series, but I think there are lots of ways to approach this project.
For instance, Janet Stone is making a series of 26 quilts using the alphabet as her starting point, but every one is totally different.
Some artists make the same design in different colors, some use the same color for different subjects. Some focus on different construction techniques. I would think that you need to decide what you want to focus on or learn about and take it from there.
I hope someone who has done some of this will chime in!