Open up the hosts file in Notepad.
You will see the line:
Let's say I want to block Website#1.com and Website#2.com.
I add this below the mentioned line:
127.0.0.2 www.website#1.com 127.0.0.3 website#1.com 127.0.0.4 www.website#2.com 127.0.0.5 website#2.com
The reason why I listed both twice (with and without the WWWs) is because the Web sites will still be accessible if you don't block both. And you can see that to add more, you just change the last number of 127.0.0.x.
Save your changes to the hosts file, and voila, you're finished!