
Step 1 - Creating a New Azure AD B2C Tenant is to ensure we have a new B2C Tenant ready to plug into our Portals.
We do this by logging into the Azure Portal HERE: https://portal.azure.com/
Within the Portal we then navigate into Azure Active Directory, this will present a view similar to the below:

Initially, this will typically show our internal Active Directory as our authentication network in the cloud.
Step 2 - If we have an existing Azure B2C Tenant, we can click the Switch Tenant option and navigate into this network of authenticated users.
NOTE: Our Internal B2B Network and our Portal B2C Network are kept in these separate Tenants so we have two separated networks of User Permissions. This concept of Tenants keeping to segregated security – often using our B2B Directories for internal and client Apps, and our B2C Directory for liaising with wider public audiences.
Step 3 - In our case, we will want to create a brand new B2C Tenant via the Create a Tenant option.
We then select a Tenant Type of Azure Active Directory (B2C) and then onto configuring the new tenant properties:

Step 4 - With these values set, we can click Create and this will start adding a brand new B2C Tenant for us to start using.
Step 5 - Our next step is then to create a new app registration in Azure B2C.
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Part 2 - Power Apps Portals - New App Registration in Azure B2C for Power Apps Portals
FURTHER READING…
Microsoft Docs Guide to creating a New B2C Tenant
Tutorial - Create an Azure Active Directory B2C tenant | Microsoft Docs