Hello! Honestly, I couldn't find it for a long time too. But after your question, I downloaded the game again and im happy to announce that I found it!
Basically its just Italy route but with some changes. You join his club, spend time with him and such. I assume you know how to get his usual route.
Back to the point. The MOST IMPORTANT part of yandere route is: during class, when there is an option to write something and Italy asks you what are you writing, choose poetry. Let him see it. Then he will ask you "Do you believe in destiny?". The answer is "Not really". And this is where it starts
After that you see "two guys with a fluffy dog" (Finland and his buddy). Choose to approach them. Youll see changes right after that choice.
After that you just hang out with Italy as usual UNTIL you come to club and see Grecia instead of Italy. Choose "I want to help".
After that just hang out with Italy until he invites you to see his project. Answer "Im busy, sorry".
No choices matter until you have "Write a love letter to someone" available. It doesnt matter to whom you write it. Just write. Thats it.
(There are also little differences between yea or no at the end)
Hope it was helpful! Be sure to say if you didn't understand something.