To get onto the true ending route, you first must complete all main endings. (which you have done) Once you do that, go to each of the 4 festival dance practices at the priest house on saturdays during the morning or afternoons. Once you have learned all the lore and done it four times, the Tai Miko ending will occur. Play through it until the end. A new cutscene will play.
After finishing the cutscene, a prompt will ask if you want to continue onto the true ending. chosing yes will play another cutscene Rinko to remember all the events prior and gathering everyone needed on the true path. This is there where you can talk to Honojou and begin collecting the talismans to finish the game.
To make this faster, use the Long Obscure Book which is purchasable on the computer. After buying it, interact with the bookshelf in the apartment. Using the book will allow you to pass as many days as you want. Just insert as many days that need to pass to skip to a new saturday.
*also, it is possible to both get the true ending and the Tai Miko ending in the same run if all other endings had been completed prior, with the Tai Miko ending leading into the True Ending*