One of the major features that shipped with iOS 9 was Markup, better known for the ability to draw on and annotate attachments of various file types that you receive in the Mail app and then send them back to the original sender, or someone else for that matter.
Despite how useful this feature is, few people actually use it, either because they don't know it exists or they don't know how to access it. In this tutorial, we'll take you through how to access it and cover some of the features it provides.