How to write a gem with rake?

I have a project based on rails. I want to generate gem file by rake and
automatically upload to rubyforge.
How can I do that?

http://rubyforge.org/projects/ropenwiki/

Go grab hoe

http://blog.zenspider.com/archives/2006/10/hoe_112_released.html

http://rubyforge.org/projects/seattlerb/

Jason