At the moment we don't have an online or desktop version of 1SE that you can use on your computer. Therefore, you can only create your mashed videos on your phone. If you don't have enough storage space to make the video, you'll need to free up some for the mashing process.
After you've created and saved your mashed video you can move it to Dropbox, Google Drive, your laptop, or any other external drive in order to have more space on your phone.