How do you complete ninja timeline?

By completing the story mode and you can play the ninja timeline

I think it would be best if you held off on the timeline until after you have completed the story. Of course you could do it while in the middle of the story as well but it's too much to keep track of. As soon as you have control of Naurto, go back to some of the areas like other villages and paths you visited during the story. There will be scrolls lying around at some of these places. The area in which these scrolls are found likely will unlock a battle that took place there. Example..if you go to Samurai Bridge, will find a scroll there unlocking the battle naruto had with Orochimaru. Also you can use the timeline to look at the Bonus requirements on each fight. Some of these bonuses are Ninja Timeline Pages..it's all a matter of completing the bonus requirement of that fight. An example of this is when Konkuro finds and fights Deidera during the beginning of the War. Using support two times in that fight (Sai) will unlock the fight Deidera had with Gaara. Hope this helps.