Hola Vivek,
If you mean, "customize" a portlet previously generated by XMLPortletFactory you should go to
Help and Support --> Wiki --> Customization -->
Eclipse edit your generated portlet (easy uh

)
If you want to develop a portlet from scratch, go to:
Liferay --> Documentation -->
Development or download examples and play around:
SourceForge.net --> Liferay --> files --> Liferay Plugins /
6.1.0.GA1 Saludos, Jack.