Go to the home page of the Youtube channel or playlist whose RSS feed you want. And copy its URL in the address bar.
A channel will look like this:
A playlist will look like this:
You want to make sure the channel URL has
/channel/ and not
/user/. If it has
/user/, try to go to the main home tab of the channel and refresh the page.
Paste that URL in the box above and press the button to get a link to the RSS feed for that page.