Click the System menu and select Custom Fields.
Press the New button.
Use the Table Name field to choose the table to add the custom field to.
Enter the label of the custom field in Field Label.
Choose the type of the custom field in Field Type.
If the Field Type is a dropdown, enter the list of dropdown values separated by commas in Field Meta. For example, if the Field Label is Color and the Field Type is dropdown, you might enter: White,Blue,Red,Green,Orange,Purple,Black. If the Field type is not dropdown, leave Field Meta empty.
Press the Save button. Your custom field(s) will now appear on the appropriate edit screens.
See this section in the Invoice Templates documentation.