How to use instafeed.js to get logged in users images

I'm building an application where users can click a button to show all of their Instagram images on a page, but in order to do that I need userId and accessToken. How do I get these?

NOTE: If it makes any difference: I'm not trying to get just my own images, but anyone who uses my app to log into their account.

I have this code:

<script>
var feed = new Instafeed ({
    get: "user",
    userId: ,
    accessToken: ""
});
feed.run();
</script>

<div id="instafeed"></div>

And I get a verification code by clicking on a link (note I removed cliend_id and redirect_uri):

    <a href="https://api.instagram.com/oauth/authorize/?client_id=CLIENT_ID&redirect_uri=REDIRECT_URI&response_type=code">Get Instagram images</a>

Then this returns a code like '1251251...' which I grab with GET, but after that what do I do? How do I get userId and accesToken from this?

I'm not sure if I'm on the right track, but any help would be appreciated!