Item Template question. How to set T4 template output as content of ProjectItem?

How can the project item output of an Item Template be set to the output of a T4 template?

I have a Visual Studio Item Template project.  Then added a VSIX project with a class that implements the IWizard interface. In the RunStarted method I prompt for parameters. Then run ITextTemplating.ProcessTemplate. How to set the output of ProcessTemplate to be the contents of the ProjectItem of the Item Template?

thanks,