Go into Azure > Subscriptions > [Your Subscription] > Settings > Resource Providers > Microsoft.AzureActiveDirectory > Register (otherwise creating B2C will give you an error).
- Create a new Resource Group called BusinessToCustomer
- Create a new resource: Azure Active Directory B2C
Step 1: Create a new Azure AD B2C Tenant
- Your company name
- yourcompanybtc.onmicrosoft.com
- Subscripton Name
- Your Resource Group (in my case, BusinessToCustomer)
Step 2: Link an existing Azure AD B2C Tentant to my Azure Subscription
- ONLY DO THIS if you did not specify a subscription / resource group when creating the resource
Step 3:
Register Azure B2C to work with Customer Portal
Automatic configuration is no longer a thing. The documentation is out of date, even though it says it was updated recently.
NOTE that this procedure is not quite straightforward. There are some things that you will need to change. For example, it says you need to check just the ID Tokens box, but you should also check the Access Tokens box or the Customer Portal will tell you that Azure B2C Sign in Failed because Implicit Authentication is not allowed.
Create user flows for sign-up / sign-in and password resets under Home > Azure AD B2C > Policies > User Flows

If you wanted to collect info during sign up, you would do it in the signup / signin flow.
Change the token compatibility settings to the second version (Azure AD B2C > Your flow > Properties > Token Compatibility Settings > Issuer (iss) claim

Azure will authenticate to this address:

If you want to add an address, do it in the B2C settings by adding an extra URI to authenticate to.
Here were my final settings upon configuration of Azure B2C with the TEST customer portal:

Registration enabled = false. That way, people can only register for the portal if there is an existing contact in our system for them.
External logout = false so it would keep them on the portal page once they sign out, instead of redirecting them to the external Azure B2C signout page.
Contact mapping with email = true means the contacts will be mapped to their unique email.
REMEMBER TO SET IT AS DEFAULT

ALSO, when you make changes to the authentication settings in the portal, you have to restart the portal (Portal Settings > Administration > Portal Actions > Restart) for them to take effect effectively. Otherwise you'll be left wondering a few hours in the future why your changes didn't apply.
STEP 4:
Customizing the sign in pages to look nice:
Go into the Azure B2C User Flows settings, select the flow, and go to Customize > Page Layouts. Here you can customize things. https://docs.microsoft.com/en-us/azure/active-directory-b2c/customize-ui-with-html?pivots=b2c-user-flow
FYI if you got through all of this, there are still a bunch of things left to configure but this post will get extremely long, so I'll leave it for next time.