Creating a smart list for a nurture program

I would like to send out an email series to inactive customers. It will include emails on day 1 of inactivity, day 7 of inactivity, day 25 of inactivity and day 30 of inactivity.

According to my understanding, the best way to do this would be to set up a nurture program that will hit all current and future customers that qualify.

Where or how do I create the smart list of leads that will be members of this nurture program?

Thanks