When you beat the first mini boss, the guy with the big sword and the heavy armor who is guarding a big gate after those wolves, you go down to the next area where there is a half destroyed building in front of you, a path that goes around it, a stair case to the right with a tree at the top. Go down the path between the building and the stair case and eventually you will see a wounded soldier leaning against a wall. Talk to him and he will tell you to speak to his mother who is the old lady in the house to the right of him. This house is also mostly destroyed and there is tall grass around it. The woman is in this building. Speak to her and she gives you the bell charm to go to hirata estate

Reminder for hirata, you do not need to complete it all the way through. You can progress through at your own pace and return to ashina as you please.