You roll a d20 and add your skill level to the roll, versus a defensive roll against your opponent. Depending on your attack, you roll Strength or Dexterity for example. Your opponent will roll the same in a defensive manner. For example, if they block they will roll Strength and if they dodge they will roll Dexterity. 

In short, it's a contested roll, your attack versus their preferred means of defense. Of course, you also need to consider other factors that might add or take away points from your roll. For example if you're caught off guard you'll get a negative modifier to your roll, while if you take your time aiming a kunai or shuriken it will take longer to execute the attack but you'll get a positive modifier to your roll.

Example:

Kai fires a Cakra Bolt at his opponent. He rolls his Dexterity +6 because that's his skill level. His opponent sees Kai aiming at him and jumps to the side to avoid the bolt, who in turn also roll his/her Dexterity to avoid the bolt. The one with the highest result wins the round. For example, Kai gets a 12 + 6 and his opponent gets a 12 + 4 because he/she has 4 in Dexterity. The result ends with Kai having gotten 18 on his attack roll and his opponent having a 16. Kai hits his target. 

I hope that makes sense. This is what I have been able to throw together from the system I've gotten explained for me.