In this example I will show you how to add the bufferapp button for sharing posts with the jetpack sharing feature.
Create / find an icon *
for the service that is a png file that is 16×16 pixels and upload to the website via media upload and use the link.
Locate the sharing code *
The code you will need for the particular service in this case it was here: http://bufferapp.com/extras/button
Work out which part of the code to use *
Add it in the right format as explained here (ignoring the [source] code which you don’t use at all: http://ryanmarkel.com/2010/08/26/adding-a-custom-sharing-service-to-wordpress-com/
Now go to the sharing settings and ‘add new service’ *
This is how it should look *