I hope this is a good enough explanation :D
Sum it up?
1. Poison a student so you can grab the tranquilizer
2. Befriend someone and ask them to follow you into the Gym Storage Room
3. Stab them with the Syringe and put them into the box
4. Tortue them at home in the basement until their Sanity is down to 0%
5. Bring them to school, give them a weapon and choose the person you want them to kill
Edit: Yes this was made in 2019 but it still works on 2020