If you are using the CKEditor, then you can do it by using a CKEditor plugin. Here is the documentation from CKEditor addons page.
If this is complicated for you, I have created a blog post with step by step information with screenshots. You can get all the info here . If you follow all the steps, your editor will look like below -
Bonus - You can even download this plugin from the blog, if you are not a programmer.