How to add any plugin to ckeditor manually?
The manual process is described on that plugin page itself, as you can see on the instruction page in first image(red circle.) 

If you click that link which you have provided in your question, you can see the Download option on that page. If you click it will download and it will open up a new pop-up window.



1. As you can see in the image. Copy that file and paste to that path.

So, I did copy and paste at at module/ckeditor/plugins/ckwebspeech

2. Now second step says, put this code config.extraPlugins = 'ckwebspeech'; So, I went to admin/modules/ckeditor/config. I did choose either Full or advanced, whichever my site is using. Clicked edit, went to Advanced options And paste the second step as shown in the step-2. Saved it ans clear the cache.


  


3. Now again went to same place at described in step-2. Instead of Advanced option went to Editor appearance and move the webSpeechSetting and webSpeechEnabled button to the custom toolbar as shown in following image.


  


4. Now if I scroll down further on the same page, I will have all option of the plugins then I enabled that plugin as shown in following image.(see second option which i checked). Saved cleared cache.


  


5. Now went to any page where I can see my editor, and, it showed up in editor.