1) Click the "Marketing" tab, then click the "Landing Pages" subtab.
2) In the Landing Page editor click on the pencil button to the left of the landing page name
3) Click on the form you want to track conversion for
4) Select the "Conversion" tab
5) Paste your conversion code into it. (you can use Adwords conversion, Facebook Custom Conversion, etc)
6) Press "Save & Push Form" button