I've seen many time travel fanfics, but this would proably work best for an oc any oc as a character in the show.

Or and oc reborn into naruto. But you still can use a character that doesn't know what's going on.

Just have them born into a time period you want the rookie 9 timeline, kakashi time line stuff like that.

And everytime that charater or oc falls asleep they end up along time in the past(how long is what's happening in your story), if its in the warring era they would "wake up" at old clan territories that's going to be apart of konha. Way back to when chraka isn't with eveeyone and just become a myth and wander around.

The Oc most likely would have slight chnages to there appearance, like hair colours turning the opposite color, maybe same with the eyes too.

they proably start doing that when they start walking.

They would stay in the timeline for max of 2 days so it passes faster. Everything aside from clothing would comething them to there timeline or back, injurys do not, and training would apply... you know experience.

If it is in the warring era you going back to, then if there is a 4th Shinobi war goodness the chaos.