Rebuild all of the things you can build in the soul room. Place a corpse in the soul extractor, extract the soul. Take the soul to the soul healer, place the soul in it and, to start with, a piece of flesh. As you expand the slots of the soul healer you can place more organs in. Healing souls gets you sin shards. You use the sin shards at the workbench in the soul room to modify organs to add red and white skulls. Create a full set of organs (including blood, fat, skin, flesh) with 3/3 red/white skulls and you will get either 4 or 7 sin shards from each soul you heal. Using the relevant sermon allows you to double this to 8 or 14. You can then start modifying corpses (after embalming) to 26 skulls.

The minor patch that just came out also allows you to craft the grave fence and sculpture in the image to give your grave 26 rating.

I have the DLC but instructions wasnt clear. I dont know how to do it tbh. Was there any guide that you followed?

Better Save Soul DLC allows you to modify the skull values of all organs except Skulls and Bones. This allows you to obtain corpses with the max value of 26.

dude how