Cortana is assistant from Microsoft just like Siri from Apple. We can make a skill for Cortana i.e Creating your own bot with in cortana so that you can use your bot through cortana by activating it using invocation name. To create SUSI cortana skill we will use SUSI API and Microsoft bot framework. First of all we will have to make a bot on microsoft bot framework and to do so follow this tutorial. Change code in this tutorial with code given below.

var restify = require('restify');
var builder = require('botbuilder');
var request = require('request');
var http = require('http');

// Setup Restify Server
var server = restify.createServer();
server.listen(process.env.port || process.env.PORT || 8080, function() {
   console.log('listening to ', server.name, server.url);
});
// Create chat bot
var connector = new builder.ChatConnector({
   appId: process.env.appId,
   appPassword: process.env.appPassword
});

setInterval(function() {
       http.get(process.env.HerokuURL);
   }, 1200000);

var bot = new builder.UniversalBot(connector);
server.post('/api/messages', connector.listen());

//getting response from SUSI API upon receiving messages from User
bot.dialog('/', function(session) {
   var msg = session.message.text;
   var options = {
       method: 'GET',
       url: 'http://api.asksusi.com/susi/chat.json',
       qs: {
           timezoneOffset: '-330',
           q: session.message.text
       }
   };
//sending request to SUSI API for response
   request(options, function(error, response, body) {
       if (error) throw new Error(error);
       var ans = (JSON.parse(body)).answers[0].actions[0].expression;
       //responding back to user
       session.say(ans,ans);

   })
});

After making bot we have to configure this bot for Cortana and to do so select cortana from list of channel on your bot from https://dev.botframework.com and add following detail shown in figure below.

Invocation name is name which you will call to use your bot. There are many ways to in invoke your skill like Run <invocation name> , Launch <invocation name>. Here is the complete list of invocation commands. How SUSI skill in cortana actually works is when user invoke SUSI skill cortana acts as middleware to send and receive responses from skill to user. After configuring your bot for Cortana test it with cortana with same hotmail account which you have used for bot framework by calling invocation name. Here is the demo video for testing SUSI Skill https://youtu.be/40yX16hxcls.

You have complete creating a skill for cortana now if you want to publish it to world select manage cortana dashboard from your bots in https://dev.botframework.com and publish it to first by filling form.

If you want to learn more about bot framework refer to https://docs.microsoft.com/en-us/Bot-Framework/index

References:
SUSI Cortana Repository: https://github.com/fossasia/susi_cortana_skill
Tutorial for bot: https://blog.fossasia.org/susi-ai-bots-with-microsofts-bot-framework/