Admin users may add and edit custom fields for entity records at "Settings" > "Lookups, Folders & Fields" > "Custom Fields".
In order to add or edit custom fields, do the following:
Edit any custom field by clicking on it or add a new field by using the “Add” field at the top of the list.
Enter a number in the "Order" field to set the order that custom fields will be displayed in your entity forms and tables.
NOTE: Plan ahead by ordering custom fields as "10, 20, 30" instead of "1, 2, 3". That way, future custom fields can be inserted between existing fields without having to edit the "Order" setting for all of your custom fields.