When you're on the Mission Maker page, scroll down to the section that says 'Social Boost Missions'. This is where you will see your most recent posts across your linked social media accounts.
When you see one of your recent posts that you would like to promote as a mission, go to the drop-down box below the post and select an action for the ambassador to do. This varies across different social networks, but is most commonly 'Like', 'Comment', and 'Repost' options.
Select one of these options, and you will be taken to the mission editor. Go through each stage and fill in any gaps which haven't been auto-populated by your post, and publish!