Navigate to Appearance > Customize > Social Media Settings.
Here, you will see a list of social networks followed by a text input box.
Type in your username for that particular network (do not insert your profile's full URL into the box unless prompted to do so).
When you enter your username within Social Media Settings, the icon of that media will appear in the header top bar and will automatically link to the URL.
To add the social icon widget to your sidebar or footer area, you can do so within WP Dashboard > Appearance > Widgets