Once the fields are added, you can enter the values while creating OR editing the invoices. Also, you need to add the placeholders for the custom fields & values with the customized template. Following are the steps to do so;

• Click the Settings page.
• Click the Invoice Templates sub-link under Invoice Settings.
• Click the New Template button and create a template (or) Choose to edit/customize the existing templates.
• From the 'New Invoice Template page'; select the following placeholders:
  • Placeholders > Invoice > Additional Fields > Custom Field 1 > %CustomLabel1%
  • Placeholders > Invoice > Additional Fields > Custom Field Value 1 > %CustomValue1%
  • Note : Follow the same to the other custom fields.
• Copy the placeholder and paste it to your preferred placed inside the template editor.
• Click the Save button

Once you add and save the placeholders to the template, make sure that you set it as a default template for the invoices.