Remove Badlink backlinks

Hello friends,
I am beginner in SEO fields. I have worked on local client web site. His website is all ready Google penalize. How to remove Google penalty, bed back links and which tool use in his process. Please tell me step by step. If you have some related blogs, please share with me.
Thanks.

You have a long way to go to take on clients, start off with disavowing all existing links on google webmasters.

Hi,
Thanks for nice tips.