Please refer to this article for an overview and step-by-step instructions on custom columns: Create Custom Columns
To create the default custom columns on evaluators' dashboards, go to the Your Evaluations tab on your own administrator dashboard to add fields.
This will set the default custom columns that evaluators (any users with assigned evaluations) will see.
If you have an Advanced license or selected this feature as part of a Standard +2 license, individual evaluators will be able to edit the custom columns on their individual dashboards as well.