Rails API with with custom strategy and omniauth-oauth2, Doorkeeper

[belgoros]

Could you suggest the way to go if I have to authenticate via a external API (like we do with Facebook or Twitter, for example) all the users who try to access a Rails API application ?  The external authentication provider provides my Rails application with its client id and secret key. I looked at many resources, rails screencasts, etc about Doorkeeper, omniauth-oauth2, jwt gems, etc and still lost how to glue all that together. So I have a Rails API as back-end, Ember JS (or what ever else) as front-end and external authentication API where all other applications should be registered. 

How to proceed, whet to use, should I create a custom strategy like it explained in Doorkeeper wiki ? Any other ways ?

Thank you in advance