There are several Chrome extensions available that provide tools to block websites. You'll need to choose one; here are a few popular options to aid your research
Site Blocker - This extension is available from Soli Software Inc. It has features for managing blocked websites on a schedule, based on keyword, and more.
Stay Focused - This extension is designed to limit access to specific websites by blocking them with the goal of improving your focus on other tasks. It's owned by Transfusion Media.
The extension will have a unique process for updating and managing your website blacklist. If there is no tutorial, look in the app settings for an area to add a website to the blocked list. You can always try a different extension if you don't like your current one.