converting an existing web application into a portlet

I'm trying to convert an existing web applicaiton into a portlet.  I'm
new to portlets, but well grounded in servlets.  I'm not quite done
with my reading and research but I was wondering if anyone could
describe the process of converting a full featured web application
into a portlet.  If you could describe the steps, the pitfalls, and
such, I'd appreciate it.

Any general advise is welcome.