Synesthesia can be navigated quite easily using the flowchart without the need for a guide.
Having said that, this guide will ensure you take the quickest route to the True End. This will also unlock all endings, achievements and tips for 100% completion.There will be a few minor scenes you may miss between the characters, but nothing of consequence.
Any route order is fine, and in some cases is enforced. This guide will take the order of Common Route, Keller Route, Isla Route, Eris Route -> True route.
COMMON ROUTE:
KELLER MINI-ROUTE:
ISLA ROUTE:
ERIS ROUTE:
TRUE ROUTE:
EPILOGUES:
Hope that's helpful!