To do this task for items but it works the same with customers/vendors/employees:
Click the Lists menu and then click Item List.
(If editing a fixed asset item, click Fixed Asset Item List.) )
Double-click any item in the list (it doesn’t matter which one).
In the Edit Item window, click the Custom Fields button.
If you receive a prompt about no custom fields, click OK.
Click the Define Fields button.
In the Define Custom Fields for Items window:
Enter the name of the custom field in the Label column and select the Use checkbox next to the Label name.
Repeat for each custom item field to add.
Click OK to close the Define Custom Fields for Items window.
Click OK to close the Custom Fields window.