This is for Android devices. If you use a search engine you can also find ways to do this on IOS using accessibility features.
Excuse the video - it was made in snapchat (duh) and I wasn't sure what I'd said in the first part. Too lazy to go check My Story.
You will need:
1 android device
1 eleastic hair tie
Stretch it round your phone or tablet so that when you are ready you can slide it over the volume button to hold it down.