On the iOS app
To preview your 1SE video, on the home screen open the project you want to mash and tap the play button at the center of the tab bar at the bottom of the project's screen.
On the Android app
To preview your 1SE video tap the big "Mash" button at the bottom of your project's screen.
This won't save your video just yet.
You will be presented with a preview of a video containing all the snippets you've added this year, but you will be able to customize it before saving or sharing it.
To learn more about how you can customize your mash, take a look at this article.