Now you are having a folder with all your files.
You are ready to test your extension.
As you can see in the upper picture, when you check developper mode appears different buttons. If you click in "load uncompressed extension" you will be asked about the location of the folder contains all files.
Once do it, will appears as a new extension in your panel and you will be able to test and debug it.
As a very light gide lines about your first extension... for example have you think about change the styles of a page getting something that you prefer instead the original? For example have you think about to get disapear the chat in Gmail?
Please check web reference about Jquery