1) Create the following custom fields in Pardot and add them to your form.

utm_campaign – Prints out utm_campaign variable.
utm_source – Prints out utm_source variable.
utm_term – Prints out utm_term variable.
utm_medium – Prints out utm_medium variable.
utm_content – Prints out utm_content variable.
gclid – Prints out gclid variable.

This is the bare minimum, you can add much more fields. Please see Native WP Shortcodes for the full list of parameters supported.

2) Go to Pardot Form and add the following snippet in "Below Form" section