Well, just copy+paste their base user URL into your feed reader. I believe the feeds are in the header tags, so your reader will find them and present you with the options.

For example, copying

https://mastodon.social/@b_cavello (the URL for the main Mastodon artist)

will allow you to subscribe to either

https://mastodon.social/users/b_cavello.atom

or

https://mastodon.social/users/b_cavello.rss

in your reader.