How to add html or javascript code

I often try to add widgets or embed code. This was difficult for me as I was lead to try gadgets by Google Help.
Creating your own Gadget was what finally did the trick. I found instructions in several places like on MyDigitalLife:
http://www.mydigitallife.info/how-to-add-javascript-or-custom-contents-to-google-sites-with-gadget/
I'm going to embed a screen shot here in case the link dies and I can't remember the steps.