Go to the show you want the ads in, and click the Campaigns tab.
From there, simply choose the episodes you want to place ads into, then choose from the available ads listed in the Pre-Roll, Mid-Roll, and Post-Roll slots.
When you're done, click Monetize. This will start to stitch your ads into the episodes. It typically takes Pippa only 10 minutes to process most of your episodes—even if you are processing a hundred episodes at once!