To configure Hybrid Azure AD Join, you will need to have a Azure AD connect server on Premise at minimum. Azure AD Connect supports Servers running 2012 R2 and above.
When you are ready, go to Azure AD portal -> Azure AD Connect. There is a link to download Azure AD Connect.
Use either express settings to automate the installation, or customize to do it manually for optional features listed below.
You will need to provide the credential of an Azure AD account with Global Administrator role here to connect to Azure AD. In my case, my owner account did not work because it is not in the format of _____.onmicrosoft.com
To create a new account with Global Administrator role, go to Azure AD -> Users and add a new user. Provide a User Name and a Name, and then click on “Roles”
Make sure you have the “Global Administrator” role checked. You can then proceed back to the Azure AD Connect installation and put in this user name and password after logging into Azure AD and changing your initial password.
Add Directory to connect to your on premise AD.
To connect to my directories for my on premise AD, I created a new AD account that is only a member of the Domain User group.
After the installation completes, you will see the on Premise users and groups synchronized to Azure AD.