Fortunately, for this game, it's very easy!

Make sure your game is fully updated to the latest version, or else you may be missing important content and fixes.

This game has a very straightforward route selection—it's literally a selection screen! There are no choices at all in this game, except for a simple choice of which route you want.

Start a new game, and play through the prologue (common route). It's fairly long, at several hours. Eventually, you will encounter the opening movie, followed by a route choice screen. This is important! Make a save file here, as you will need to come back to this point often for the later routes (there is no route select on the title screen). If you forget to make a save, load up chapter 1_08 from the Scenarios screen and skip to the end.

Got sent back to the title screen instead of a route selection? Try asking around on the official Discord for technical assistance.

Note that once you complete a route, a notification will appear saying that a new scenario has been unlocked. Each route has an additional one to two epilogue stories that must be started by clicking Scenarios from the title screen, navigating using the tabs on the right, and manually starting the story. From there it will progress through all afterstories to the end.

The epilogue chapters are 2_13, 3_16 - 3_17, and 4_21 - 4_22.

P.S. A tip: this game has a lot of random background chatter that doesn't show up in the textbox. However, pretty much all of it does show up in the backlog, so open that up if you're curious what was said.

You can do any route order you want and it will be fine. Feel free to choose whichever you want; none are necessary prerequisites for any others. However, once you complete all three, something interesting will happen.

Once you have completed all routes, DO NOT FORGET to go back to the route selection one last time: a new route will mysteriously appear—the Grand Route. I don't believe any notification of it being unlocked shows up either. This is the longest and most epic route, so buckle up! It acts essentially as a "true route," putting together everything you've experienced so far into one massive journey. Good luck!