The first step is to find the profile ID of the user you want to follow via RSS. To do so, pull up any user’s profile on Google+; the long number at the end of the URL is the profile ID. For example, the URL for my Google+ profile is https://plus.google.com/111668640050600254907/. 111668640050600254907 is my profile ID.
Plug in that profile ID at the end of http://plusfeed.appspot.com/ – so the RSS feed of my profile would be http://plusfeed.appspot.com/111668640050600254907. Add that into Google Reader or another feed reader to stay updated with that user’s public stream.