1. From the Field tab of the Form Builder, click the Add Custom Form Field button or Edit User Defined Field
2. Click the Tooltip Configuration Tab
3. Modify the English and Spanish Field Hint
4. Click the Save Field button when finished.
5. Tooltips will automatically be added to the field when the user highlights the field.

This will add a "talk bubble" that gives the user a pop-up description of the field.  The tooltip looks similiar to the image below: