arcgis desktop - How to Calculate fields for attribute table using ModelBuilder?

I am working with the ModelBuilder tool in ArcMap. I have created a simple model that takes the Strip Map Index Features as an input. I have added fields to the model to produce more fields in the attribute table. When I run my model, it works as the original Strip Map Index Features tool should work, however, in the attributes table, I see my fields that I added, but they are not being populated / calculated.

I am not sure how to calculate / populate these fields?

I have searched around the net for solutions but am not getting too may concrete answers as I am still pretty new to using ModelBuilder.