save as soon as you get to the first poetry writing task.

(make sure you look at the game files as you play because secret extra poems appear there as you play)

playthrough one of the girls routes. before you go to school on Monday for the festival, reload the first save. ( don't worry your time with the first girl still counts)

repeat this with the other two girls and only then go to the festival and get Sayori.

play through the rest of the game however you like, it wont matter for the ending.

i never see this ending played through, honestly i only ever see Yuris route played which is a shame because i cry at the end card for this ending. every time. i love this game. i love this concept. i love the characters. i love the soundtrack. i love it. ^-^