If you've just finished the first chapter and are wondering now how to unlock other ones, it's actually way simpler than in any other Steins;Gate entry.

Just press Start button in the main menu and you'll be directed to a sort of flowchart where you'll be able to choose with buttons which route you want to read next. You'll need to read certain routes first in order to unlock next ones later, but that's about it.

Plain and simple so don't worry about your planned 100% completion.