To add custom field, go to System Setting > Custom Field then select Field Belongs To, Field Type, Field Name, Grid (Bootstrap Column e.g. 6) - Max is 12, Field Values (Separate By Comma) and check Validation, Visibility and then click on Save button. You can view this added record in the Custom Field List on the right side of the page.

Add custom Field image

To edit custom field click on Edit icon and to delete custom field click on Delete icon present in the custom field list.