How to add a button on the toolbar

I'm trying to simply add a button on the toolbar but I can't seem to undestand how to do it. I tried following this example: http://blog.tommed.co.uk/2009/09/07/how ... -v3-plugin
But it won't work! The toolbar is displayed according to the config.js, but the new button won't even appear! Could anyone clearly explain to me how to do it? Or show a sample. It's just a button that will do something simple when clicked that I need.

Is it just a button name folder and plugin.js inside it I must create? And then configure config.js file for it to appear? Please help! Thanks in advance!