There is an article available at Liferay Wiki to help you develop a Struts Based Portlet using there Plugin SDK, but chances are pretty high that the article alone wont suffice.
Here are a few problems you might face when you develop a Struts based Portlet.
1. You might get errors for ActionForward, ActionMapping and so on.
2. Your project will have compilation problems.
Here are a few simple steps to create a Struts Based portlet: