Take a peek at your custom dashboards to automatically see who of your subscribers is becoming more or less engaged with your organization and newsletter. The components titled More Engaged Subscribers and Less Engaged Subscribers will tell you who has been opening emails more frequently than in the past and who has been slacking a bit.
But if you would like to upload those subscribers to segments, this way is more efficient:
By going to Insights > Subscribers > Individual Subscriber Metrics and updating the Reporting Period in the top right corner of your screen to whatever time period you'd like to see statistics from, you can see how your subscribers have been receiving your newsletters. Click the orange filter button, export your file to a CSV, and access it in the Exports tab. Once you open the downloaded file, you'll be able to see both regular and unique Open and Click-Thru Rates for each of your subscribers.
To upload the subscribers who fit your requirements (e.g. if you want a segment containing all those who have not opened a newsletter in the past 6 months, remove all the subscribers from the Excel file who have opened newsletters), simply add subscribers to a new segment within Cerkl. Don't forget the format for CSV files - just full name and email address in the two columns!