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: https://www.youtube.com/channel/UCrTNhL_yO3tPTdQ5XgmmWjA
A playlist will look like this: https://www.youtube.com/playlist?list=PLY9cZ8nX4xmlpd1RE8_toocbU9cND7U-A
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.