Modifying Headings for automatic numbering

Question

I am using Word 365 and I am trying to modify my headers such that they can display as the following;

1. Chapter One

[Body Text]

1.1 Section One

[Body Text]

1.1.1 Subsection One

[Body Text]

1.2 Section Two

[Body Text]

I want the numbering to automatically update so that I can later use the Table of Contents to seamlessly put the sections together with the page numbers. I've tried modifying the heading option but I cannot find the option to create a multilevel heading. The following is the closest I've gotten to finding the option I'm looking for.

Answer 1

Fine point of jargon... Originally in your question you said "Headers."

What is the difference between headings and headers?

You may want to look at : How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly

Once you set  up your numbering using the method shown by Stefan or using Shauna's methods, it is important  to apply the numbering by applying the associated styles, not using the buttons on the Home Tab for numbering.

Answer 2

Click the Multilevel List button, and select the option to use 1, 1.1 etc., with Heading 1, Heading 2 etc.: