Modify the page with CSS Form Builder directly
You would probably find that the easiest process would be to use CSS Form Builder to modify the page directly. If that page already has a form on it, CSS Form Builder will import all the ids for those fields, allowing you to simply specify what type of form element they need to be. Then add any additional fields, and design everything.
When finished, CSS Form Builder will replace the existing form with the new one you created. In most cases, this will also maintain the submit/insert functionality that may have previously existed.
We have a How To that you may find useful called Modifying an existing form with CSS Form Builder on the CSS Form Builder Support page css-form-builder/.