How do I add CiviCRM fields to a layout box / container in a form created using webform-civicrm?
when you're on the Webform tab - you can drag and drop your fields into the Layout Box - here's what it will look like:



And re: the second part of your question - no - you don't need Drupal Form Builder. When you've downloaded and enabled webform_civicrm module - you'll see a CiviCRM tab (on the far right side) and this is where you tell the module to pull in certain Fields from your CiviCRM: