Firstly, the storm bar needs to be filled, the small yellow bar underneath the health bar. You'll also need to be at about 40% health.
For this, you need to be on a team. So you can startup a VS AI match and set it as a team battle. You charge by holding Y (if you're using a controller) and finish the opponents off. While on a team, you need to have access to the "linked awakening". So, for sake of ease, something like Naruto and Sasuke.
Below is a video for clarification: