To create an Insta-Mash, mash a video like you would normally (play button). Then tap the "Insta-mash" option on the top. This will prompt you to log in and then allow you to mash all your posts together.
It'll take a minute or two for us to download all the posts and throw them together but it'll be worth it! Once it's created you can save or share the video. Hope you enjoy it!