The plugin which I use is Simple Custom CSS. It is very easy to install and set it up, as its name implies.
It is over at: http://wordpress.org/plugins/simple-custom-css/
1. Go WordPress Dashboard.
2. Scroll down to Plugins.
3. Click on Add New.
4. Type Simple Custom CSS into the search box.
5. Click Search Plugins button.
6. A new page will appear and Simple Custom CSS is first one on the list.
7. Click Installed. Then click Activate Plugin.
8. It is now listed among other plugins in the Installed Plugins page.
9. If you need to add any code in the newly installed custom CSS file, go to Appearance.
You should see the words "Custom CSS" among Theme, Customize, Widgets, Menu, etc.
10. Click on Custom CSS and a new page should appear.
12. Then click on Update Custom CSS button.
That's all.