If you go to the recollection room, you'll be told how to get different endings per route. To find a route, just follow the coloured arrows. To get the final route, you need to get an ending for each route and then go back and try another route instead of resetting each time.
There is 7+ major endings, and they are pretty straightforward in that three endings are "do the demon realm route", three endings are "do the mind control society route", and 1+ ending is "do the wriggle route".
Once you have an ending, the NG+ equivalent unlock the second/third ending of a route (so you need ng+ and then ng++ for second/third demon realm route endings, etc).