curl https://www.opscode.com/chef/install.sh | sudo bash
That's it. This can be put in any instructions, such as a README or someone's blog, since the logic is in the shell script. Provided you download the script using https, the file has standard levels of authentication and encryption protecting it from manipulation.
This is obviously a shell script, if you're really concerned about the argument that it may contain nefarious activities within, you can easily review it before you run it.