1. Find yourself a module you like, for example: MMM-IP
2. Top right on the github repository you see a green button saying “clone or download”. If you click it you get the url for this repository. In this case it is https://github.com/fewieden/MMM-ip.git
3. Open a terminal on your pi (either directly or using SSH) and type cd ~/MagicMirror/modules
4. Clone the module to your pi by typing git clone https://github.com/fewieden/MMM-ip.git. This downloads the repository and extracts it into a new folder.
5. Enter the new directory cd MMM-ip and run npm install (in this case it is not required … the readme should tell you to do so)
6. Edit your config file and add the code for the new module as the readme instructs to.
7. Restart your magic mirror process and check what happened.

Sidenote: If you run into trouble with your config file, you can always paste the content into http://jshint.com/. If everything is fine it will only show this which is fine:

One undefined variable
64	module

var config = {
        port: 8080,
        ipWhitelist: ["127.0.0.1", "::ffff:127.0.0.1", "::1"],

        language: 'en',
        timeFormat: 24,
        units: 'metric',

        modules: [
                {
                        module: 'alert',
                },
                {
                        module: "updatenotification",
                        position: "top_bar"
                },
                {
                        module: 'clock',
                        position: 'top_left'
                },
                {
                        module: 'calendar',
                        header: 'US Holidays',
                        position: 'top_left',
                        config: {
                                calendars: [
                                        {
                                                symbol: 'calendar-check-o ',
                                                url: 'webcal://www.calendarlabs.com/templates/ical/US-Holidays.ics'
                                        }
                                ]
                        }
                },
                {
                        module: 'compliments',
                        position: 'lower_third'
                },
                {
                        module: 'currentweather',
                        position: 'top_right',
                        config: {
                                location: 'New York',
                                locationID: '',  //ID from http://www.openweathermap.org
                                appid: 'YOUR_OPENWEATHER_API_KEY'
                        }
                },
                {
                        module: 'newsfeed',
                        position: 'bottom_bar',
                        config: {
                                feeds: [
                                        {
                                                title: "New York Times",
                                                url: "http://www.nytimes.com/services/xml/rss/nyt/HomePage.xml"
                                        }
                                ],
                                showSourceTitle: true,
                                showPublishDate: true
                        }
                },
          ]
        };

/*************** DO NOT EDIT THE LINE BELOW ***************/
if (typeof module !== 'undefined') {module.exports = config;}

var config = {
        port: 8080,
        ipWhitelist: ["127.0.0.1", "::ffff:127.0.0.1", "::1"],

        language: 'en',
        timeFormat: 24,
        units: 'metric',

        modules: [
                {
                        module: 'alert',
                },
                {
                        module: "updatenotification",
                        position: "top_bar"
                },
                {
                        module: 'clock',
                        position: 'top_left'
                },
                {
                        module: 'calendar',
                        header: 'US Holidays',
                        position: 'top_left',
                        config: {
                                calendars: [
                                        {
                                                symbol: 'calendar-check-o ',
                                                url: 'webcal://www.calendarlabs.com/templates/ical/US-Holidays.ics'
                                        }
                                ]
                        }
                },
                {
                        module: 'compliments',
                        position: 'lower_third'
                },
                {
                        module: 'currentweather',
                        position: 'top_right',
                        config: {
                                location: 'New York',
                                locationID: '',  //ID from http://www.openweathermap.org
                                appid: 'YOUR_OPENWEATHER_API_KEY'
                        }},
                {
                        module: 'newsfeed',
                        position: 'bottom_bar',
                        config: {
                                feeds: [
                                        {
                                                title: "New York Times",
                                                url: "http://www.nytimes.com/services/xml/rss/nyt/HomePage.xml"
                                        }
                                ],
                                showSourceTitle: true,
                                showPublishDate: true
                        }
                },
        };

/*************** DO NOT EDIT THE LINE BELOW ***************/
if (typeof module !== 'undefined') {module.exports = config;}

1. open a terminal type in cd ~/MagicMirror/config to change directory
2. nano config.js
3. After you made your changes press ctrl+x and confirm
4. change directory to parent directory cd ..
5. start the mirror npm start