Microsoft Multi Tenant App as an Individual Developer - Azure Active Directory

When you said "multi-tenant application", it means you need to own an AAD tenant to create it.

I assume that you are already an individual developer registered to Microsoft. What you need to do is Create a new tenant in Azure Active Directory. Sign into https://portal.azure.com/ with your individual account to do the above operations.

Please note that your account is a personal account currently, you need to follow the the guidance the portal shows (if it shows) to convert it to a work account as well and then you can create the tenant.

After that you can create multi-tenant application in Azure AD.

Select one of the options to make it as multi-tenant app.

UPDATE:

"Starting November 9th, 2020 end users will no longer be able to grant consent to newly registered multitenant apps without verified publishers" alert means normal user cannot do user consent but admin still can do admin consent.

You need to do the admin consent for custom's tenant by using admin consent (see this answer). It doesn't matter that if they don't do the admin consent before using your application. When the first time an admin from other tenant tries to sign into your application, he will be required to do admin consent as well.

Please refer to auth code flow to learn about the authentication process. Using /common or /organizationare both OK.