Zero Saga(Haruno route)
We might have met somewhere
Once you finish this route instead of New Game option you should get Heaven Saga option.
Heaven Saga(Ichimonji route)
I dont know you
After finishing Heaven Saga you will have an option of starting Sky Saga.
Sky Saga(Can be considered True Route)
There is no option in Sky Saga, just enjoy your read.
In order to get 100% CG after finishing every Saga you have to enter Start Game and read Ex Scenario 1 (Haruna's Sex Scene) and Ex Scenario 2 (Ichimonji's Sex Scene)
With that you should have 100% CG and almost all Jack's Journal Pages.
In order to get all pages you have to enter Heaven Saga and read it (or skip it) repeat untill you have all the pages you need.