All that you have to do is establish the authentication keys, then synchronize your fields and all of your Lead fields, even custom ones, are available to use in Joomla. We were able to have lead forms up and running on our website in about 30 minutes. Pretty slick, JoomTiger.
Even though we gave this 5 stars, we do have a suggestion for the next version:
Allow defaults for field values to be set in Joomla, for both visible and hidden fields, in each module deployed on the site. For instance, we use a "Lead Type" custom field where we can indicate whether the Lead is for web design, custom development or one of our own Joomla extensions. We'd like to be able to set this field in each module we use, then hide it from the user. That way the module that is published on web design content pages can send the appropriate Lead Type automatically, without it needing to be a user selectable field.
All in all, great work. Wish there was something that worked this smoothly for Joomla and vTiger user authentication and synchronization! (Hint, hint)