Then I suggest you read tutorials about this. It also requires that you learn JavaScript, so you need to learn some as well. There are great tutorials and books out there, all you need is research.
What you need to do later, is find a way of replacing text within the chat window with an img-tag that represents your emote. But you should not try to copy-paste a solution, no one wants to use an extension where the developer doesn’t know how to solve bugs.
Good luck! 