I did this a few weeks ago, so am writing these instructions from memory.

Prerequisites:

• A yahoo account
• A running FusionAuth instance (localhost is fine)

Steps:

• Go to the Yahoo! developer network and create an app.
• The redirect URI for Yahoo is https://<your instance>/oauth2/callback
• Save off the provided Client ID (Consumer Key) and Client Secret (Consumer Secret).
• Then go to FusionAuth and create an OpenID Connect Identity Provider: <your instance>/admin/identity-provider/add/OpenIDConnect
• Put the Client ID (Consumer Key) and Client Secret (Consumer Secret) into the Client Id and Client secret fields, respectively.
• Uncheck Discover Endpoints. Manually configure the endpoints:
  • Set the Authorization Endpoint to https://api.login.yahoo.com/oauth2/request_auth
  • Set the Token Endpoint to https://api.login.yahoo.com/oauth2/get_token
  • Set the Userinfo Endpoint to https://api.login.yahoo.com/openid/v1/userinfo
• Set the Scope to openid email profile and any other scopes you might need. (I was unable to find an authoritative list, but here's info about the mail scopes.)
• Update the Button text and Button image as needed.
• Enable it for applications as needed.
• Save the Identity Provider.