@pastelpunkbandit@hellsite.site you take someone's profile url and add .rss on the end like https://hellsite.site/@pastelpunkbandit.rss

like any good site, mastodon will advertise it's existence in the page sourcecode, if you go "view page source" it has
<link href='https://hellsite.site/@pastelpunkbandit.rss' rel='alternate' type='application/rss+xml'>

a good rss reader should be able to parse that automatically, just by giving it https://hellsite.site/@pastelpunkbandit

but be warned about trying to rss social.pixie.town/@f0x.rss :))