To start intense training you want to start on an unstable surface. Try to rock backward and forwards to work on flexibility. Making sure you are at your level trying to get the maximum stretch.
As an athlete, you need to have full flexion to prevent injury and help with speed. After you're able to do this front to back take it side to side slowly and controlling your motion.
To get a full range of motion it takes time to stretch the structure of the ankle. If you want maximum benefit as an athlete you want to work your body from the bottom all the way up.