Source: http://lifehacker.com/5559317/learn-to-make-your-own-chrome-extensions
Chrome extensions are actually pretty simple at their core; they're basically just web pages, except they also have access to aspects of your browser, like tabs or history. Like any other page, you can use javascript and style sheets to really customize the look and feel all you want.
Tutorialzine's instructions will walk you through creating your first extension, that will act as a popup with a very pretty display of an RSS feed on it (pictured above, using theirs). It's simple, but the tutorial is pretty great—it walks you through each step of the process and makes it easy to understand which parts of the code are linked to certain attributes, as well as providing helpful links to certain sections of Google's documentation on extensions. If you've ever wanted to make a Chrome extension, it's a pretty good place to start. Hit the link to read more, and let us know what you think of the process in the comments.