Like I mentioned in my previous post, you won’t be able to use any old phone. OnePlus One details which phones are eligible, but you can use whatever you want to smash it. Some of the suggested items are a hammer, baseball bat, screwdriver, and a blender. As far as what phones you can smash, there are 25 in total, and the majority of them are pretty current. One exception is the Galaxy S III, which is nearly 2 years old, but it is a very popular phone. They also go as far back as the iPhone 5.
Assuming you have an eligible phone, you do need to signup and hope that you are one of the 100 selected. If so, you just need to smash your phone and share it to the world via YouTube.
source: OnePlus Smash