First, make sure you are in top 128 in a points ranking and not just battle. Now, you can go to Space-Time window and in Strongest Ninja- check first option on left of screen- Lookup Group Sectional- it will open the group you are fighting in, if you counted into Strongest competition. Click on your group in a chart on screen to check who will be your opponent.
Then just show up for your fight on certain time- countdown on screen. Not showing online and entering your battle in a set amount of time will make you lose.