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.
To learn more about how you can customize your mash, take a look at this article.