In case you already have web application (war), just put it in deployment folder ( “deploy” folder for tomcat and “deployment” folder for jboss )
After it is deployed, Liferay will see it as a portlet. To use portlet, go to your liferay homepage > add a page. Then select manage that page, your portlet will be located in “undefined” category. Add it to your page and you are ready to go.