It's easy! Add a subscription in your RSS reader to
channel with the name of the channel you want to subscribe to.
Want an RSS feed of a playlist? Grab the playlist ID from the playlist URL, and point your RSS reader to
playlist_id with the ID of the playlist.
Want to subscribe to a channel that doesn't have a legacy-style channel name? Grab its channel ID (e.g.
UCV9_KinVpV-snHe3C3n1hvA) and point your RSS reader to
channel_id with the ID of the channel.