Open the quicktags.js file in the wp-admin folder of your WordPress installation. Insert your version of the following into the list of buttons where you want your own quicktag button to appear.

edButtons[edButtons.length] = new edButton('ed_name' // the name used with the setup ,'name' // the name that will be displayed on the quicktags bar in the Admin section ,'' // the opening tag the button will insert ,'' // the closing tag the button will insert ,'x' // the accesskey to activate this button from the keyboard );