Start by visiting the page from the uPortal 101 pre-conference seminar. There you'll find some preliminary steps in order to get Eclipse and some required plugins up and running. Next steps are:
- start up HSQLDB
- unzip a uPortal distribution, build it and then run "ant initportal" to deploy it and initialize the database (give the correct url of the HSQL instance you started above)
- create a "Tomcat project" in Eclipse, call it, say, "uPortal" and copy the whole WEB-INF/classes folder under your Tomcat's webapps/uPortal/WEB-INF/src folder and webapps/uPortal/WEB-INF/classes
- go back to Eclipse and start Tomcat through the sysdeo plugin
- create a separate project for your channel and build it into the WEB-INF/lib folder of your uPortal project in Eclipse.
I think this is pretty much it. I've listed the above from memory so I'll be glad to help you further if anything's missing.
Nuances: this is more complicated if your channel includes media that needs to be installed into the uPortal media directory.